As an observant Hindu, Brij Sharma considers cows sacred. He believes the gentle creatures are helpmates to human beings, and it would be as unthinkable for him to eat beef as it would for a cowboy in Montana to eat his own horse.

 Not surprisingly, Mr. Sharma, an electrical engineer for Boeing in Seattle, says that for years he never set foot in a McDonald's restaurant. But in 1990, when the fast-food chain announced with great fanfare that it was switching from beef fat to "100 percent vegetable oil" to cook its French fries, Mr. Sharma joined the legions of Hindu Americans and vegetarians who began venturing into McDonald's to nibble what they believed were vegetarian fries. Mr. Sharma's teenage son even took a job at McDonald's last year, and drawn by the generous employee discount, the Sharma family consumed countless bags of fries. So Mr. Sharma said he was horrified when he opened his India West newspaper in April and read, "Where's the Beef? It's in Your French Fries." He and other American Hindus were outraged to learn that McDonald's French fries are seasoned in the factory with beef flavoring before they are sent to the restaurants to be cooked in vegetable oil.

 Now Mr. Sharma is one of three plaintiffs representing the Hindus and vegetarians of America in a lawsuit filed on May 1 in Seattle that accuses McDonald's of deliberately misleading its American customers.

 "I feel sick in the morning every day, like I want to vomit," Mr. Sharma said in a recent interview. "Now it is always there in my mind that I have done this sin."

 The news ricocheted to India, where restaurant windows were smashed, statues of Ronald McDonald smeared with cow dung, and Hindu nationalist politicians called for the chain to be evicted from the country. In Fiji, a majority of Hindus and vegetarians told pollsters they had heard about the beef in the fries and stopped eating at McDonald's.

 McDonald's, a corporation that prides itself on catering to the culinary requirements of ethnic and religious groups in its restaurants overseas, says it uses no animal extracts in the French fries it sells in India and in Fiji, where nearly half the population is of Indian descent. A test of the French fries by an Indian organization after the rioting confirmed the claims of the company, which says it is going forward with plans to expand in India.

 McDonald's says it never said that the French fries sold in the United States were vegetarian. The marketing campaign proclaiming the switch to vegetable oil in 1990 was "all about healthy hearts and eliminating cholesterol," Walt Riker, a spokesman for McDonald's, said in an interview last week. "We certainly don't market ourselves as vegetarian."

 He said McDonald's added beef flavoring to the fries before they were flash frozen, and complied with Food and Drug Administration regulations by saying that it included "natural ingredients," without specifying what they were. And although McDonald's may re-evaluate its labeling policies, Mr. Riker said, it does not intend to alter its recipe.

 "These are the ways the fries are made in the U.S., and we don't have any plans to change," Mr. Riker said.

 Burger King and Wendy's restaurants do not use beef products in their French fries, their corporate spokesmen said in interviews on Thursday.

 Vegetarian groups had suspected there was beef flavoring in McDonald's French fries and petitioned the company and the Food and Drug Administration for full disclosure of ingredients with no success. Fast-food restaurants are highly secretive about their recipes, and it was only after the lawsuit was filed that McDonald's spokesmen widely acknowledged the beef ingredient.

 "They would post these lists of their ingredients in their stores, but nowhere did they ever publicly admit that beef flavoring was used in the fries," said James Pizzirusso, who founded the Vegetarian Legal Action Network with other law students at George Washington University.

 "Corporate America needs to pay attention to consumers who avoid certain food products for religious or health reasons, or because they have allergies," he said. "They say they are complying with the law in terms of disclosing their ingredients, but they should go beyond the law." Vegetarian advocacy groups claim to represent as many as 15 million Americans. And while those groups are accustomed to confronting American corporations, the lawsuit is a watershed for the Hindus in the United States. Mostly first- and second-generation immigrants from India, with a smattering of American converts, they are estimated to number more than one million people. Until now, they have put far more effort into educating their children and building temples to perpetuate their religion than into pressuring the federal government or industry to accommodate their customs.

 The lawsuit came about when the vegetarian law students connected with the outraged Hindus.

 The students at George Washington University had drafted the legal complaint as a project for a class on legal activism, and were looking for a lawyer to file it. In Seattle, Harish Bharti, a Hindu lawyer, read about the secret ingredient in the article in India West and decided to sue.

 India West heard about the ingredient from Hitesh Shah, a Los Angeles software engineer and a strict vegetarian. He had sent an e-mail inquiry about French fries to McDonald's, and received a reply from a customer service representative who wrote that McDonald's used "a minuscule amount of beef flavoring as an ingredient in the raw product."

 Mr. Bharti called Lige Weill, executive director of the Vegetarian Awareness Network in Knoxville, Tenn. Mr. Weill had already succeeded in getting the Wendy's chain to stop putting gelatin (made of animal collagen) into the sauce on its Fresh Stuffed Garden Veggie Pita.

 Mr. Weill introduced the students to Mr. Bharti, who had been in the news for suing corporations and defending battered women. Mr. Bharti also teaches trial skills at the Gerry Spence College for Trial Lawyers in Wyoming. "I see this as a fight for the dharma," said Mr. Bharti, using the Sanskrit word that loosely translated means "ultimate principles."

 "Eating a cow for a Hindu would be like eating your own mother," he said. "People have told me, 'I would rather die here than go to McDonald's.' " In the last 10 days, he has filed additional lawsuits against McDonald's in California and in Canada.

 In the Indian-American neighborhoods of Chicago and Houston, in the sari shops and vegetarian restaurants, many Hindus said they had heard about the lawsuit, but not all said they agreed with it.

 Parag Gandhi, 32, the manager of the Taj Sari Palace in northwest Chicago, said he considered himself "a McDonald's man," and thought the Hindu plaintiff foolish. "I don't think Ronald McDonald walked up to him and made him eat the French fries," Mr. Gandhi said. "People should know that if they are eating at a place that serves meat products that they have to be more than careful if they don't want to eat meat. Come on, I mean it's McDonald's."

 At the Anand Bhavan Vegetarian restaurant in Houston, Mahendra Jagirdar, an engineer, said he had stopped eating at McDonald's when he heard about the fries. "I'm a pretty strict vegetarian," he said. "That's why I like a place like this where I don't have to compromise."

 As for Mr. Sharma, the plaintiff in Seattle, he is seeking ways to cleanse himself. "I am now planning to go to India to take a dip in the Ganges," said Mr. Sharma, the grandson of a Hindu religious counselor to a maharajah. "I am thinking that it should reduce my sin. But the damage is already done."