AFTER ALL THE auditors I've suffered through, I could slit my throat that I didn't think of it myself," said Anita Loos.

It as late in 1974 and New York's liveliest gossip, who wrote "Gentlemen Prefer Blondes" along with 400 movies, was filling me in on the events of her week, a Sabbath custom as sacred to us as church is to the righteous.

"It's something going on down in a rehearsal room of Joe Papp's Public Theater. Once they finish it, it's going to be gorgeous. Months ago Michael Bennett got together with some his gypsies and taped stories of their lives. They're putting them together in the form of a musical show audition, one of those calls that are so painful to endure. Even the portions I've seen are dazzling, straight out of life, pure theater. It's going to be the musical of this year and next year, whenever they open."

That was the first I heard of "A Chorus Line," which reaches the Kennedy Center Opera House Thursday night following a flossy benefit preview the evening before.

Loos was right. In time "A Chorus Line" would win the Pulitzer Prize, the drama critics' citation and nine Tony awards. Joseph Papp, who nurtured it so long in one of his Astor Place workshops, would state that its first two years got his new York Shakespeare Festival out of debt for the first time in 20 years. From three American companies, one in London, another in Australia, Bennett's income has zoomed to $90,000 a week from his various roles as director, choreographer, co-author and co-producer.

That Bennett doesn't keep that weekly $90,000 for himself is one of the aspects that has made "A Chorus Line" exceptional from the start. He shares it with his choreographic associate, Bob Avian, and 27 members of the original group whose lives form the basis of this study of today's gypsies.

"Gypsies," in theater terms, are dances who spend 20 to 30 years in the choruses of musical shows.If they're playing in New York - or looking for work - they often live in "cold water" flats, meaning rent-controlled tenements. When they're "on the road," they split into smallish groups, from two or three to six or seve, to share furnished apartments, taking turns at K.P. and cost-cutting. Watching them disgorge from trains or planes with luggage, packages, children, birds, cats and dogs, you know that a musical or ballet company is about to open.

The gypsies are a special breed of theater, and it was from some of them who'd been in her won shows that Loos knew what was happening at Astor Place. Jobs are scare, rivalries strong, yet all share winning and losing, recognizing that luck and whim are part of the system. It's too tenuous a life for pettiness. The theater's survivors don't keep up only with their peers. Chorus people, electricians, wardrobe women, hairdressers, a.s.m.'s (assistant stage mangers), writers, musicians and stars are all part of the family, and its grapewine is endless. In their retirement at Genesee Depot, Wis, Alfred Lunt and Lynn Fontanne kept up with many who were associates 30 or 40 years before.

Starting as a kid dance in "West Side Story," Buffalo-born Bennett, now 35, has risen to choreographer and director. In the past 10 years he's been nominated for 11 Tony awards. "Promises, Promises," "Coco," "Company," "ollies," "Seasaw" and the nonmusical "Twigs" have earned him acclaim and income.

Aware that his success came at least partially through his dancers. Bennett was dissatisfied with being "the American dream itself and I started to spend a great deal of time alone. Then I told myself I was too young to be a hermit. I'd better get back with the people."

To Bennett "the people" meant the dancers with whom he'd spent all his working life. He got the idea of gathering a group of those he knew to talk about their lives. After theater, of course. At midnight Jan. 18, 1974, he met with 27 dancers and started his tape recorder. "It had been years since they had thought about their childhood and how they had become dancers." That night and a later session produced 30 hours of taped memories and introspections.

The two taping sessions clarified in Bennett's mind the point that had eluded him in his hermit period. In the impersonal chorus lines dancing behind the glittery star performers are individuals of rich talent, stern discipline and arresting lives. This became the theme of "A Chorus Line" and even those with no theatrical involvment have perceived the applicability of anonymity to their own lives.

One of the taping participants, Nicholas Dante, had said he'd tried writing as an alternative to his erratic dancing career. Bennett gave him the tapes and from theem came a script without music or lyrics.

Enter Joe Papp.

Joseph Papp created New York's free Shakespeare Festival in 1954 and his production activity has made him the most resourceful of new theater leaders. He kept New York from tearing, down the vast old library on Astor Place and prevailed on private donors and foundations to create performing spaces there of varying sizes. When he took over Lincoln Center, he warned that unless he received enough money to run it, he'd drop it. That's just what he did this summer.

"A way of life" is how Papp describes theater as a career. "I'm not interested in the old syndrome of hits and misses," he'll tell you. "I am interested in letting things grow, making it possible for plays to happen, slowly, gradually, naturally and I want theater for the poor people I grew up with.

"When Bennett came to me with his idea, it was the exacxt sort of thing that inspires my respect. The idea was to develop the outline, pay, the performers $100 a week while they were doing it." Papp provided working space, cash and, most important, no deadlines for public performances. "Work on it," he said, and then let them alone. He'd wander in occasionally and was pleased but not impatient. Without that attitude," A Chorus Line" never would have happened.

James Kirkwood, the son of silent film stars Lila Lee and James Kirkwood, had been a performer who turned to writing when jobs were scarce. Tallulah Bank-head was one of his believers and when "There Must Be a Pony" was published, she encouraged him to dramatize it. Myrna Loy acted it; later came the respected "Good Times/Bad Times." Bennett brought him into the collaboration.

"It turned out be a writing job from beginning to end," Kirkwood has stated. "Nick, Michael and I would get together and play "What if?" What if one of the dancers got hurt? What if the director had had an affair with one of the dancers? We used our imaginations on the real-life stories the dancers had taped. Some people became other people.Thanks to Joe Papp, we had the luxury of having the time we needed to work it out."

For music and lyrics, Bennett turned to two other old friends. Marvin Hamlisch, who'd been a dance musical arranger but never the composer for Broadway musicals, had a fantastic night at the April '74 Academy Awards when his "The Way We Were" won both best song and best score categories and "The Sting" won in the best adaptation division. Having played rehearsal pianos, Hamlisch knew just what sounds were needed and that "change, step, step, point, point, point, flick, step, kick, touch, change, walk, walk" rhythm was in his fingers. Despite all his movie offers. Hamlisch wanted a stage musical "so bad I could taste it."

Lyricist Edward Kleban had worked with Goddard Lieberson producing original cast records for Columbia. To learn more, he had studied lyric writing under Lehman Engel, the conductor-composer-historian, at the MBI Musical Theater Workshop. Kleban's lyrics, so vital to the show, stemmed from the crash course he gave himself studying dancers, their habits, attitudes and vocabularies.

It took the better part of a year to make the endless choices musical-show creation demads. Though mirrors are a dramatic device in Robin Wagner's production, the designer claims that "a simple white line is the only scenic essential in this show, which could be perfomed in a parking lot."

"The luxury of time was my joy," says costume designer Theoni V. Aldredge. "Rehearsal clothes are different to design, must be real but theatrical and here express the personality of each performer who tells his won story. I had time to study the characters as they developed."

Lighting plays a subtle role, harsh, white work light becoming the reality of the story on this bare stage. Colored lighting accompanies internal thoughts. Tharon Musser advanced the art by striking ingenuity for the shifting moods and in time complex, computerized lighting boards would be developed for the move to Broadway.

While the first sound heard is an old piono's, the orchestrations, by Bill Byers, Hershy Kay and Jonathan Tunick for an unseen 18-piece orchestra, were considered a challenge in "sound design" by Abe Jacob, whose equipment can make the dazzling finale sound like "an MGM-type musical sound track."

Long before the Off-Broadway opening and its previews, there were run-throughs in Astor Place and the grapevine began to crackle with excitement. But Papp was determined not to open until Bennett was fully satisfied.

One member of those early, informal audiences played a critical role. Neil Simon's actress-wife Marsha Mason, complained that Cassie, the character who once had had an affair with the director, didn't land a job. "You are saying to us," she told Bennett, "that if we don't make it, we can't go back. You can't take starting again away from people." Bennett took her advice. Cassie gets the job.

From the official Off-Broadway opening, April 15, 1975, "A Chorus Line" has been setting records. It moved uptown to the Shubert but played for months of previews thare while a musicians' strike delayed its formal opening.

When it was decided to form three new companies to tour, Bennett rehearsed all three simultaneously in the cavernous underground of old New York City Center.

The one the Kennedy Center is getting is "the international company," most traveled of the lot. It tried out in Toronto in April '76, opened to huzzahs in London's historic Drury Lane three months later. Because foreign performers are limited to six-month runs in London, that group next played Baltimore, as a hypo for its revived Mechanic Theater, and Miami before settling down for three months in San Fransco. That's where hte gypsies will be boarding their planes today for arrivals tonight at Dulles.

The mirrors won't be flying with them. There are two sets, leap-frogging cities, slow freight to Philadelphia, where the Kennedy Center company move after closing here Nov. 5.

Papp is too much the realist to expect the show's vogue to last forever. "I see another two years ahead just like these last two," he says, "and that means it will be supporting everything we do in New York into 1979. These things bappen once in a lifetime."

One of the works financed through "A Chorus Line" profits will be at the National for a month starting Oct. 10, Ntozake Shange's "For Colred Girls Who Have Considered Suicide/When the Rainbow Is Enuf."

"A Chorus Line" on film is certain, Universal paying $5.5 million for the rights, the same sum fetched by "Myu Fair Lady," highest ever paid for a Broadway creation. What will it be like? "Not a copy of the stage original," declares Bennett, who will direct.

Already legendary, "A Chorus Line" came along when the American musical was in sharp decline. With no big new ones on the immediate horizon, will "A Chorus Line" be a link to the future or the end of the line?