By the time he reached his sixties, Dominic F. Antonelli Jr. had amassed one of Washington's biggest fortunes -- more than $100 million, by some estimates. The one-time parking attendant built a parking company, Parking Management Inc., accumulated downtown real estate and gradually replaced his garages and parking lots with office buildings.

Then, over the last decade or so, he risked it all by bankrolling several other developers, investing in scores of their projects and guaranteeing bank loans on their behalf.

Many of those projects did not work out. Antonelli has defaulted on much of his $548.8 million in loan guarantees, and on Friday he will seek protection from creditors in personal bankruptcy while he tries to reorganize his finances, his lawyer, Roger Frankel, said yesterday.

Documents prepared by Antonelli's accountants and recent interviews with his associates reveal a man who did business in unconventional ways -- leaving legal control of developments that he funded in other people's hands, entrusting huge sums of money to others and relying on unwritten agreements. That could make it extraordinarily difficult for many of his creditors to recoup money they are owed.

Unlike most developers, Antonelli did not assemble a substantial real estate organization of his own as he expanded. For the most part, he depended on his partners to run things.

"He trusted in the ability of these people," said John P. Kyle of Vector Realty Group Inc., a friend of Antonelli. "He didn't look that hard at the downside."

Some of Antonelli's partners said they never imagined the full scope of his investments. The many properties in which Antonelli owned partnership interests were valued at $1.8 billion and carried loans of almost $1.3 billion as of last April. In addition, Antonelli personally owned real estate he valued at $91.1 million, which carried relatively little debt.

Based on what little they did know, some of Antonelli's partners had long doubted his ability to oversee his investments, they said. His personal staff numbered less than half a dozen people, sources said. "He could not keep track," one of the developers Antonelli supported said. "He had no organization to monitor what was going on."

Altogether, Antonelli invested in about 165 partnerships, which control apartments, hotels, office buildings, housing developments, shopping centers and thousands of acres of undeveloped land as far away as Florida and Nevada. He had advanced $185.2 million to various partnerships and business partners as of April 30, 1990. He had pledged his personal fortune to 36 lenders to secure borrowings of $548.4 million for himself and his partnerships as of Aug. 31, documents show.

Antonelli's partners included:

William I. Darter, president of the Tysons Corner-based Churchman Corp., with whom Antonelli invested in Days Inns and other budget hotels from Bucks County, Pa., to Coral Gables, Fla.

Investors Management Group Ltd. of Elkridge, Md., headed by Douglas Nyce, with which Antonelli invested in commercial and residential development projects, many near Richmond.

Hal Boles, Walter M. Cheatle and Stuart H. Gary of Huntmar Associates Ltd., also based in Tysons Corner, with whom Antonelli invested in Loudoun, Fairfax and Prince George's county office developments.

Richard Weiser, head of McLean-based Tri-Equity Group Inc., with whom Antonelli invested in land, shopping centers and other projects from Montgomery County to Orlando, Fla.

Weiser's story offers a window into Antonelli's world. Seven years ago, Weiser, a commercial real estate broker, began developing real estate on a modest scale. Two years later, Antonelli agreed to put his financial clout behind Weiser.

Being affiliated with Antonelli, Weiser said in a recent interview, has been "the happiest work experience ... that I've had in my entire career in the real estate business."

At a time when it seemed that real estate values would only go up, it was "relatively easy" to get land loans that required almost no out-of-pocket payments by Weiser and Antonelli, Weiser said. Banks would make loans that covered the full purchase price of land and paid their own interest charges, Weiser said.

The relationship was in some respects a role reversal for Antonelli, who benefited earlier in his career from associations with Charles E. Smith, a bigger developer with more financial clout, and Kingdon Gould Jr., a wealthy descendant of 19th century financier Jay Gould.

In the 1980s, eager to book hefty lending fees, bankers competed to do business with Antonelli. They were content with Antonelli's personal guarantee because they considered him to be financially unshakable, like Donald Trump, Conrad Cafritz and a select group of big-name developers.

Last April, Antonelli was predicting that investments in real estate partnerships would generate $24 million in cash payments to him during 1990. But cash appeared to be flowing out faster than it was coming in. In 1989 and early 1990, Antonelli was writing checks for millions of dollars a month to prop up faltering projects.

Moreover, bigger bills were coming due. According to Antonelli's April financial statement, he expected $80.4 million of loans to expire within a year. In the carefree days of the real estate boom, banks routinely renewed loans, but by last spring, most banks had grown reluctant to grant extensions.

Recognizing that he was throwing good money after bad, Antonelli curtailed the flow of funds to his projects and prepared to attempt a sweeping financial restructuring, sources familiar with his finances said.

Even as Antonelli began to confront the severity of his problems, a few lenders maintained faith in his creditworthiness. Early last February, before it was seized by bank regulators, National Bank of Washington lent Antonelli $10 million based on his personal guarantee. Two months later, First American Bank lent Antonelli $6 million based on his guarantee.

With so many players and projects involved, any restructuring of Antonelli's finances was bound to be difficult. But two hallmarks of Antonelli's business could make it even more difficult for banks to collect on the personal guarantee that they once accepted eagerly.

First, there is the structure of his investments. Most developers play the role of general partner, which gives them control of their partnerships' real estate. In contrast, Antonelli is merely a limited partner, a noncontrolling minority investor, in many of his partnerships -- perhaps more than half of them, according to one source.

In those cases, Antonelli's creditors may be powerless to seize the real estate. The best they can do may be to sell Antonelli's interests in the partnerships -- and minority shares of partnerships that own troubled property are relatively worthless, real estate executives said.

Then, there is the trust factor. Asked to describe Antonelli, people who know him reply almost universally that his word is as reliable as a written contract. Antonelli apparently felt the same way about business partners: He lent them millions of dollars and often accepted an oral promise of repayment instead of a written contract, sources said.

For example, Antonelli had advanced about $19 million to Huntmar Associates and assorted Huntmar projects as of last April, the financial statement said. One Huntmar source said he is not aware of any written agreement promising Antonelli repayment. "The oral understanding was he would be repaid his money out of the profits on the projects when they got sold down the road," the Huntmar source said.

The absence of written agreements could hinder creditors if they attempt to recoup some of the $185.2 that Antonelli said his partners and partnerships owed him last year.

And in many instances where written agreements exist, the money Antonelli advanced to others may be irretrievable. It has already been spent or tied up in distressed property, observers said.