A FAMILY OF megamillionaires begins a test of wills in a Manhattan courtroom this month, battling for control of one of America's great fortunes. At stake is the $400 million to $1 billion in the estate of J. Seward Johnson Sr., a dapper playboy and heir to the Johnson & Johnson medical supplies empire, a man who lived in the style of a 19th century monarch thanks to the profits from products like Band-Aids, Tylenol, Q-Tips and Johnson's Baby Powder. On one side is the attractive Polish ,emigre and one-time maid who became the third (and final) Mrs. Johnson in 1971 when he was 76 and she was 34. Barbara Piasecka Johnson, known in the family as Basia, stands to inherit control of virtually all of that vast estate under a will her husband signed 40 days before his death in May 1983.

Challenging the will are Johnson's six children, all of them already multimillionaires thanks to trust funds their father established for them in 1944. Of the six, only J. Seward Johnson Jr., the sculptor, whose works include "The Awakening" on Washington's Hains Point, is named in the will. Beyond the fees he would receive as an executor and trustee, he is bequeathed $1 million and a house on Cape Cod. The five other children, born during Seward Sr.'s first two marriages, are left nothing. The will also ignores the Harbor Branch Foundation, the oceanographic research institute the senior Johnson founded and funded during the last dozen years of his life.

"It's going to blow high, wide and handsome," Seward Jr. once predicted, in one of his few comments about the proceeding, "and it's gonna be a lulu."

Neither the widow nor the children would consent to interviews about the case, but the story of their dispute and what led up to it is laid out in thousands of pages of documents filed in New York's Surrogate Court in connection with the impending probate trial. These court papers, which include depositions given by Basia and all the children, offer a family portrait of a rich old man and the younger woman who delighted him, of six often quarrelsome children and of indulgent life styles that are a glaring counterpoint to the conservative and publicity-shy firm that generates their vast wealth. Beyond the seven main characters is an eclectic cast of acquaintances that reveals the family connection in politics, art, science and show business. A list of the 450 people believed to have seen or spoken with Seward Sr. during the last three years of his life -- which Basia prepared under court order -- ranges from two former secretaries of state (Cyrus Vance and Alexander Haig) and a former presidential national security adviser (Zbigniew Brzezinski) to Seward Sr.'s foot doctor and his massage therapist. It includes several Rothschilds, a Rockefeller, a Roosevelt, Gregory Peck, Chief Justice of the United States Warren E. Burger, former president of Italy Amintore Fanfani and Barbara Sinatra, Frank's wife, who is said to have once borrowed a lavish rug from Basia.

Claiming their father hallucinated and relived events from his youth in his final weeks, the children insist he was unable to understand the complexities of the 45-page will he signed on April 14, 1983. One of their lawyers contends in an affidavit -- which is based on information supplied by his client and others -- that Basia and Nina Zagat, an attorney for Basia and Seward Sr., exercised "fraud, duress, coercion and undue influence" on a cancer-ridden and feeble old man in shaping the will. An attorney for the widow and Zagat counters in a similar kind of affidavit that every testamentary decision Seward Sr. made was logical and reasonable, and that he was alert and lucid nearly to the end of his life. In support of this, he submitted a physician's affidavit attesting to Seward Sr.'s mental competence signed the day he executed the will.

The children, says the affidavit filed by their attorneys, are fighting not for themselves, but for Harbor Branch Foundation, to which Seward Sr. donated $130 million during the last dozen years of his life and which would have received $70 million in a will its patron signed a few days earlier than his final testament. The affidavit goes on to outline the children's claims that their stepmother was able to "enchant and captivate" Seward Sr.

The widow counters, through affidavits and motions filed by her attorneys, that her husband considered his children wastrels who squandered their trust funds and brought shame to the family with their outrageous conduct. Their father's largess had already allowed them to live like royalty, one lawyer's affidavit argues. "Six nabobs . . . whose greed has led to this litigation," one affidavit asserts of the children. The lawyers declare that each would be worth more than $110 million today had they never invaded the principal of their 42-year-old trust funds.

The lawsuit bares many intimacies and ironies, including sordid details of some of his children's divorce suits, of alleged suicide attempts and of long- forgotten quarrels.

"Combat is delicious, though," Seward Jr. once said, speaking about his art. "It's a wonderful way to sort out the truth."

Jennifer Johnson, Seward Sr.'s youngest daughter, recalled her father as a strong-wlled but shy man whose conservative exterior masked a rebellious streak. "I think part of him was a naughty little boy, way inside," she testified in her deposition. "He would kind of giggle and say,'Ah, they're not going to get me."

With the fortune he made when the company went pumlic in 1944, Seward Sr. drew up a grand scheme for his life. He told his attorney of the time, James Pitney, that he wanted to set aside one-third of his fortune for his family, another third for charity and, with the remainder, he planned to "live it up."

His expensive divorce settlement, careless investments and high living seriously eroded Seward Jr.'s fortune, Basia's lawyers say. In hisown testimony, Seward Jr. estimated his wealth t $23 million and acknowleged he is the poorest of Seward Sr.'s children. James Johnson, the younger son, who is a New Jersey farmer and artist, and Jennifer Johnson estimated in their depositions they had personal fortunes of about $100 million each, making them the richest of the six. The other children, Mary Lea Richards, Diana Firestone, who operates stud farms in Waterford, Va., and Ireland, and Elaine Wold, testified of personal wealth ranging from $58 to $75 million.

"HE SAID that he was very impressed when he learned that I saved my money earned by hard work in order to undertake my studies, that he respects me very much for this reason, that I remind him of the American pioneers," Basia Johnson testified of her elderly husband at a deposition taken last October. Speaking through a Polish interpreter, whose translations she occasionally corrected, she said that Seward Sr. had tracked her down soon after she'd quit her post as a maid in his New Jersey home and offered her "a special kind of job."

Basia had been in the United States barely a year. She'd arrived in New York on Labor Day 1968 with $100 in her pocket, not long after graduating from Boleslaw Beirut University in Wroclaw with a degree in art history. Polish friends helped her find work at the Johnson estate and in 10 months, she testified, she saved $4,000.

She was taking an English course at New York University when Seward Sr. invited her to his New Brunswick office and sent a limousine to chauffeur her. His plans to build an art collection, she testified he told her, required someone with her background in art history. He also wanted her to learn scuba diving -- they would go to the Bahamas in November for practice -- so she could work for the oceanographic institute he wanted to establish. Of course, her English classes were important, he agreed, but why not pursue them with a private tutor? Naturally he would pay all the bills. He gave her a check for $2,000 that day.

Soon she was traveling with him to the Bahamas and to Europe. He apparently set her up in an apartment on New York's Sutton Place, the children's lawyers allege in an affidavit. During her deposition, Basia's attorneys refused to let her answer questions about how she came to live at 45 Sutton Place South. That's where Joyce Johnson, Seward Jr.'s second wife, testified she first encountered her father-in- law with his new companion. "It was an awkward occasion," Joyce Johnson recalled in her deposition. "He marched us into the bedroom for no reason at all and sort of, like a teen-ager might do, said, 'Look!' and there was a large nude over the bed and it looked like a little intimate boudoir."

After marrying in 1971, the rich old man and his young bride traveled around the world on their honeymoon and settled down to a life of splendor. Of course, they had their little quarrels, like any married couple, Basia conceded. She scolded him for not taking his medicine and he yelled at her for squeezing the toothpaste tube in the middle.

Choosing Princeton for their summer home, they built a $25 million estate and named it Jasna Polana, after Leo Tolstoy's home. In the style of landed gentry, they raised livestock in the 140- acre compound and made their own butter. But this was no ordinary farm -- it cost more than $2 million to run the estate in 1983. The mansion is flanked by a $78,000 orchid house and an air- conditioned dog house complete with kitchenette and a special passageway leading into the mansion with heated bronze steps. In the only interview she's ever given, published in the Times of London in 1983, Basia denied reports Jasna Polana had 40 rooms. "Actually there are very few," she said. Her Flemish tapestries, she explained, require very large rooms. "Besides," she added in majestic afterthought, "I like to have air."

Together, Basia and Sew filled Jasna Polana with an estimated $50 million worth of art treasures -- 18th century French furniture, Renaissance paintings, Impressionist works and, of course, the tapestries -- she preferred 16th century courtly scenes.

They owned airplanes, yachts, an island in the Bahamas, a home in Italy and a place in Florida. She sometimes outbid the Getty Museum on her purchases and made art world history, most recently in 1984 when she paid $4.8 million for a chalk sketch by Raphael.

When Seward Sr. married Basia, he promised her a $10 million trust fund and a $250,000 gift at his death. Soon after the wedding, the Johnsons met Nina Zagat, a young associate with the New York firm of Shearman & Sterling (and author of a well-known New York restaurant guide), which had been engaged to do some of Seward Sr.'s legal work. In 1973, Zagat began helping to draft a number of the wills that enlarged Basia's share of the estate. Finally, in his April 14 will, one of three testaments and one codicil that he signed in a 56-day period in 1983 during his final illness, Seward Sr. left virtually all his money in Basia's control, with Zagat and Seward Jr. named as executors and trustees. Beyond $5,000 and $10,000 bequests to 28 of his longtime servants and the legacy to Sewfe interest in the bulk of the estate. In addition to the income from these funds, which might reach $50 million a year, the will authorizes her to borrow unlimited amounts from the principal and to set aside $20 million for "descendants of her father." The will further al- lows Basia to select the charities to receive the estate after her death, a provision her attorneys contend was contained in a number of other wills.

The children are excluded, as they were in many prior wills. "I have intentionally made no bequests in this will to my descendants," Seward Sr. wrote in the document. "I created a substantial trust for each of them during my lifetime."

Thomas Ford, a lawyer with Shearman & Sterling who helped Zagat draw up many of Seward Sr.'s wills and codicils, says in an affadavit that the explanation for those increasing bequests to Basia is simple. "Mr. Johnson loved and trusted his wife and wanted to provide for her to the maximum extent possible without incurring estate taxes." Besides, Ford argued, allowing Basia to decide which charities would receive the family fortune would put some restraints on Seward Jr., whose conduct was once again troubling his father.

To support their differing interpretations of Seward Sr.'s will, both sides point to a final bitter argument between father and son, one that took place in a Boca Raton hospital while Seward Sr. was receiving a blood transfusion two months before he died. Fearful that Basia and Nina Zagat were in complete control, Seward Jr. testified, he asked his father to resign and turn over the leadership of Harbor Branch, duties Seward Jr. otherwise would have assumed after his father died. Recalling the awkward scene, Seward Jr. said his father seemed to be thinking, "This is a hell of a time to be bringing this up."

Basia testified that she arrived in the midst of the dispute and found both men angry and agitated. She intervened, she said, and the following day, with Basia's help, Junior got his way. He quickly scrawled a resignation letter, had his father sign it, then told his stepmother, "I hate that old father of mine," according to Basia's testimony.

The children's lawyers argue that the incident illus- trates Basia's ability to enchant and captivate her dying husband, while Basia testified the incident convinced her husband Junior would destroy the foundation and led the older man to revise his will in Basia's favor.

"I know my husband's dreams," she told the Times of London. "That is why he want to leave it all to me."

THE CHILDREN speak of dreams, too, portraying themselves as the rightful guardians of their father's legacy.

Although some of them had seen him only intermittently before his illness, they made frequent visits to his Fort Pierce winter home during the last five months of his life. He was essentially bedridden, attended by a retinue of private-duty nurses and by doctors who helicoptered in for house calls. The prostate cancer that had been discovered in 1981 had spread to his bones and it was clear death was approaching.

Far from disappointing and embarrassing him, the children insisted in their depositions, he took pleasure in the activities that filled their lives. He delighted in the successes Mary Lea and her third husband Martin Richards had in producing theatrical hits like La Cage aux Folles and Sweeney Todd. Diana and Bertram Firestone's Waterford stud farms interested him and, rather than disapproving of a suit she is fighting in New Jersey against the trustees who control the 1944 trust established by her father, he was quietly supportive. James and Jennifer were active members of the Harbor Branch board. And, they said, he was especially proud of Seward Jr.'s sculptures and of the Johnson Atelier, the bronze art foundry Junior established where innovative casting techniques attract a clientele of internationally known sculptors.

Ironically, it was their stepmother who helped unite the sometimes fractious family. Seward Jr. once called her "Basia the peacemaker" for her efforts to resolve their quarrels. Other testimony sug- gested that Basia's vola- tility was sometimes distressing to her husband. Basia shrieked and screamed at the servants, Mary Lea Richards testified, and was known to fire people for setting a soup plate down too hard or serving wine incorrectly. And when her rages focused on their father, several of the children testified in depositions, she called him "stupid," "gaga" and "crazy." Basia denied ever calling her husband stupid and said when she used the word "gaga" it was meant only as a joke.

A WEEK AFTER Seward Sr.'s funeral in Fort Pierce, Sew convened a meeting of the children in New York's posh Metropolitan Club. Junior had learned from Nina Zagat how his father's estate was to be divided. The children were stunned and disappointed by the news. "It doesn't seem like a will that my father had a part of," Diana Firestone testified she thought. Together, Seward Jr. testified, they decided any will that ignored both his beloved Harbor Branch and his own descendants while it provided for "a bunch of Polish people he hardly knew" couldn't possibly reflect their father's intentions.

Only James Johnson hesitated briefly. "How can we be sure that this was not Daddy's wishes?" Mary Lea recalled him asking. Seward Jr. responded with his own question. "Jimmy," he asked his half-brother, "do you think your father was not senile?" James replied, "No, I think he was senile," Mary Lea testified.

"This will solve a lot of problems," Nina Zagat remembered J. Seward Johnson Sr. saying as he sat in the dining room of his Florida home on April 14, 1983, and signed his last last will and testament. Far from it. So far the 45-page document has gobbled up 15 percent of the manpower of Manhattan's Surrogate Court for more than two years, and the pro- bate trial is still ahead.

For Zagat, the will potentially means a fortune. Ac- cording to an affidavit by the children's attorney, her fees as executor and trustee may be $12 to $13 million, an amount the attorney contends is "highly questionable." The will provides that the executors, including Zagat (whom Seward Sr. identifies as "my friend"), be paid according to the percentage provisions of the New York law. Zagat testified that she told Sewesser fee; he told her he wanted her to receive the larger sum the law provided, she testified.

In all, the forthcoming trial is shaping up as a great piece of legal theater. The law firms involved, Sullivan & Cromwell for Basia Johnson and Nina Zagat, Milbank Tweed, Hadley & McCloy for the children, and Dewey, Ballantine, Bushby, Palmer & Wood for the passed-over Harbor Branch Foundation, are among the most exalted -- and most expensive -- of New York's society law firms. The trial is expected to last six weeks, but the case may drag on for years. The jury's verdict is subject to appeal and since a probate jury considers only one will at a time, there's the remote possibility of more than 20 separate trials -- should each will be considered -- before a settlement is reached.

Like Seward Jr.'s tortured giant at Hains Point, this case is burdened with questions of meaning. What would Sew make of this dispute that's putting the most intimate details of his life on file in a courthouse records room? How much family honor will be left when they've finished clawing at each other's dignity? Is it worth the millions in legal fees and the staggering emotional toll that it's costing to prove each other unworthy of Seward Sr.'s legacy? Why doesn't Basia just give them each, say, $25 million and settle this public dogfight?

Perhaps one of Seward Sr.'s grandsons guessed the truh about the family in a letter to his grandfather. "It's too bad about all this goddamned money," he wrote. "If it were not there maybe we could all be a closer family."