The movies have stars. Grand opera has its prima donnas. The fashion business has Halston.

Yesterday Roy Halston Frowick, at 43 undeniably the heaviest hitter of American fashion design for half a decade, dropped in on Washington for a day. And it was no simple guest appearance for this man, but an agenda with all the splash and flash that a major, high-profile business can command.

This is the man who says there are no fashion designers, only fashionable clients ("Fashion is made by fashionable people," he says), with a client list that reads like American fashion royalty - Jackie Onassis, Babe Paley, Lauren Becall, Marisa Berenson. He is a walking commercial enterprise. If you can wear it, put things on it, smell it or sleep on it, Halston has designed it - in versions that total, with his fragrances, over $100 million at retail.

He arrived yesterday - flying first-class, of course - with an entourage of eight and two more shifts of six each arriving later, for press interviews and a television appearance. But mainly he came to present his collection at the Care Hall at the Embassy of Iran last night, which is under the sponsorship of Garfinkel's.

More than half of his entourage were New York models for the show. "I must have models who know the clothes, know where the hooks and eyes are, where the snaps are and know how to walk in them," said Halston as he lit up a cigarette and learned back in the limousine, one of three sent by the store to pick up the group at the airport. "It just makes this kind of a trip so much easier."

It made things easier, too, he said, to bring his own assistants, the dressers and pressers, all at Garfinkel's expense; and his game plan for the day included seeing reporters only at 10-minute intervals at the store and talking to store customers not at all ("I have staff to do that," he confided).

The strategy went slightly awry when he stepped off the elevator on the third floor, guided by store executive Hanne Merriman, and a customer in corduroy tapped his shoulder.

"I just have to tell you that I wore a white gown of yours as my wedding dress and I loved it," said Jule Konick, an executive aide at George Washington Medical Center."But I had to buy a 6 and I really wear a size 2."

With a sweeping gesture Halston assured her that if she made the request, he would indeed whip something up in her size. "Oh, I'll certainly remember that the next time I'm in Saks," she replied.

"No, no," Halston and Merriman shouted in unison, ever the entrepreneurs. "At Garfinkel's!"

Garfinkel's was underwriting the major expense of the 12 models, average cost $500 per day each, plus traveling expenses for the whole crowd. (The Shoreham chipped in the rooms.)

One customer not so satisfied yesterday was Mrs. Myron Levin of Highland Park, Ill., who was denied permission by a New York court to sue for $10,000 for mental anguish over a $680 Halston custome-made dress than she claimed was "a disaster."

Said Halston: "The price she paid doesn't sound like a made-to-order dress, but maybe it was. Sometimes people's measurements change and the clothes don't fit when they arrive." Later he added, "Please say that I am sorry and I will be happy to take the dress back."

Palling around with Elizabeth Taylor (who was due at last night's ball), and Iranian Ambassador Ardeshir Zahedi is a far cry from Des Moines, where Halston was born, and Evansville, Ind., where he grew up. Once out of Chicago Art Institute, he worked in a department store there and created a few hats in his apartment after work. His first client was Fran Allison of Kukla, Fran and Ollie and celebrities have never stopped coming.

While creating millinery collections for Bergdorf Goodman he met Jackie Kennedy and created the pillbox that became her signature at the time and his most famous hat design.

Halston since has made dresses just as famous, just as influential as the pillbox, including the first Ultrasuede dress ("We've made hundreds of thousands, 75,000 or more of just one style," said Halston, who still includes Ultrasuede dresses among his best sellers).

What's coming next, he says, are "dresses, dresses and dresses." Noticing without humor that an interviewer was wearing pants, he added, "Women who wear pants are discovering how flirtatious and feminine skirts can be."

He approves of jeans - for others, not himself - as a great way to show off the figure. He doesn't wear them, though he looks good in them. "I don't find them comfortable. The legs are too tight," he says. What he does wear is a custom-made Italian-indigo-blue suit when it fits the occasion.

Last night he was wearing a 2-year-old tuxedo, pretty darling for a man who designs menswear too. "A woman who has a 2-year-old dress might feel a bit old fashioned," he said, but "a man has the luxury of having a black tie that lasts till he wears it out."