ATLANTA (AP) _ In a letter mailed Friday to 900 advertising and public relations firms, the head of the Atlanta Olympics appealed to the companies' patriotism to refrain from ``ambush'' marketing.

Ambush marketing, in which companies make unauthorized use of Olympic emblems or themes, has been a growing problem at recent Games. With 1996 marking the centennial of the modern Games and offering live prime time television coverage in the United States, some fear the problem could be greater than ever in Atlanta.

Federal law gives the U.S. Olympic Committee the right to control who uses Olympic symbols. The privilege usually goes only to corporations that pay large amounts to become sponsors.

Olympic officials have in some cases taken legal action to thwart unauthorized ads. But the letter from Billy Payne, president of the Atlanta Committee for the Olympic Games, tries a different approach.

Payne notes that corporate sponsorships are paying a large share of the costs of the 1996 Games, as well as paying for the training of U.S. athletes. If the value of the sponsorships is diminished by ambush ads, he says, such corporate support could dry up.

``As a leader in the advertising industry, I look for your support, whether it is verbal, written, or moral, in creating a collective voice against parasite marketing before it damages the Olympic movement,'' the letter says.

``Only with full understanding and an ethical commitment from corporate America can U.S. teams and tomorrow's hopefuls be assured of the support they need to seek athletic excellence.''

Payne wrote that lawsuits will be filed if persuasion fails.

The letters are a pre-emptive strike against ambush marketing, which generally begins on a large scale around the time of the Games, ACOG spokesman Scott Mall said.

``We are appealing partially to people's patriotism,'' Mall said. ``It's not a matter of the market share of McDonald's vs. Wendy's. When it happens it's not corporation vs. corporation _ it's in effect robbing the United States Olympic Team.''

Jeffrey White, general manager of the J. Walter Thompson ad agency's Atlanta office, said he doubted the letter would influence many people.

``My feeling is the ones that would feel they shouldn't do ambush marketing wouldn't do it, and they don't need a letter to tell them that,'' White said. ``Those that will _ a letter is not going to be that meaningful to them.''

The huge audiences that are drawn to the Olympics are an irresistible target to advertisers, including those who cannot afford to become an official sponsor, White said.

``Clearly, you shouldn't violate the Olympic trademarks and logos that are reserved for sponsors. Other than that, I think there are a number of advertisers that will say this is an advertising opportunity like anything else and will take the fullest advantage of it.''