At the swanky Prime Rib, an expensive downtown eating establishment, a sign posted on the door reads, "Coat Required." Owner Nicky Blair, sitting at the bar during the lunchtime rush hour, says the sign is to "keep the bums out of here."

Other Washington restaurants have similar policies designed to maintain a certain sartorial decorum, they say, and attract what they consider to be a suitable clientele. But yesterday, four of Washington's most exclusive eating establishments were hit with a lawsuit charging that their policies of requiring men -- but not women -- to wear jackets while eating is discriminatory.

The lawsuit was filed by a group of George Washington University law students, who contend that the four restaurants -- The Prime Rib, Sans Souci, il Giardino and Le Provencal -- were violating provisions of the D.C. Human Rights law that prohibit discrimination on the basis of sex or personal appearance in restaurants.

"It's a frivolous suit," said Mario Fazo, owner of il Giardino. "If you have a high-class restaurant, spent a lot of money for decorations and paintings and have the best chefs in Europe, then would you have people come in slovenly dressed? Nobody has to be dressed like a king," Fazo said but he added, "I think the judges will throw it out of the courts."

The students who filed the suit, Gil Karson, Wayne Kaplan and Eva Booker, said they had conducted a three-month investigation in which they and some friends, all wearing trousers and a dress shirt, tried to eat at various downtown restaurants. But, they said, the men were refused admittance because they were not wearing jackets. The student lawsuit asks that the restaurants pay a nominal amount in damages, remove their dress-code signs and advertise new nondiscriminatory policies. Any money award would be donated to a charity, the students said.

Students contended that the discrimination against men because of the way they dress could lean to discrimination based on race and other reasons.

A former waiter at a downtown restaurant who was wearing a white hood "to shield his identity and protect him against retaliation" was introduced by the students at a press conference. He said that the restaurant where he formerly worked -- not one of the four being sued -- the policy requiring jackets was used arbitrarily to keep certain people out of the restaurant. He said the exceptions included instances where the host knew the person without a jacket or the person was well known.

Law professor John Banzhaf said the lawsuit filed yesterday in D.C. Superior Court was conceived in a legal activism class he teaches. He warned other restaurants "not to continue their policy [of requiring jackets only for men] or you may be the defendants in the next lawsuit." He said the students' investigation was continuing.

The D.C. law invoked to challenge the restaurants' policies prohibits discrimination because of race, color, religion, sex, personal appearance or sexual orientation.

Around town yesterday, restaurateurs expressed skepticism at the students' lawsuit.

"Everything is discrimination, everything is discrimination," said Georges, maitre d' at Le Provencal, 1234 20th St. NW. "For 30 years, every place I work requires a jacket. I don't say they don't belong here, this door is here for everyone. If I go places without a jacket, I will feel uncomfortable, that's the way I look at it."

Hostess Kathy McGraw said Le Provencal "is too expensive for students" anyway, with lunch ranging to $15 a person. At the Prime Rib, at 2020 K St. NW, owner Blair motioned a reporter to look for himself to find male customers without jackets. The search was fruitless.

Well, Blair said after glancing around the dining area, if the reporter had been there the day before, he would have seen that things were different. Some of the diners, he said, were wearing sweaters.