It was the third time she'd been to McDonald's that Sunday. She didn't bother to open her game stamp until she got home. It was Stamp 737, a picture of an egg with a hat on it. And something about $100,000.

"Look here, Mother," she said.

"Oh Jean, you just won a dozen of eggs, that's all," said her mother.

But no. Margaret Jean Tansill of Hyattsville, mother of five, who hasn't had a vacation since her honeymoon 25 years ago this June, had just become the nation's first big winner in the McDonald's "Quality in the Bag" giveaway game.

That's when the screaming started.

"I just screamed and went to my knees," she said at a company ceremony yesterday in what downtown Washington knows as the Sans Souci McDonald's. "It's scary, you get so excited. I don't remember what I did. First I yelled a lot and then I would just sit and think of all the things, how it will affect your life. I'd just sit there and feel great."

There is one other $100,000 stamp floating around the McDonald's restaurants of America. Chances of finding it next time you go there are 1 in 686,210,000. After 16 tries the odds would be down to a mere 42,888,125. Both of the $500,000 grand-prize stamps in this $20 million lottery are still at large. It is also possible to fill sheets of less magical stamps to collect prizes ranging from the half-million down to $5, a Big Mac or a glass of orange juice.

"I never bothered much about the stamps," Tansill said. "The children have been working on it, so I always ask for 'em. I even got one that morning when we stopped in for coffee after mass. I went back later with my nieces and nephews. Then for some reason I went again that afternoon." Her blue eyes widened behind the steel-rim glasses. "But from now on I will never, never give any away! Not Green Stamps or anything!"

Turning the stamp into money was the first problem. Major winners must be verified at the McDonald's redemption center in Beverly Hills. Tansill didn't trust registered mail, so she spent $300 to have her brother-in-law, Robert Stemple of Clarksburg, W.Va., drop it off in a detour from his skiing trip to Denver.

"I've never flown," said the West Virginia farmer's daughter. "We insured Bob for $300,000. When he called and said it was good as gold, we went nuts."

With the check in the bank, she consulted the accountant who does the books for the family home remodeling business. Figuring in her eight dependents, he estimated the tax bite at $20,000.

Four of the five children are in college. There's John, Ionia, Paul, Sibby and Mike. And one granddaughter, Connie. "Just the oil and gas for their cars is quite an item," she said. "They said I should get a new car, but I already have a new Olds, which I love."

Her husband, Edwin, ran a wood splinter through his right eye last fall, losing the sight in it. That took him off the job for three months. He has only just come back to work. "I feel like I'm a broker's husband," he laughed.

"We're taking our time with it," she said calmly, "it" being the family code word for the $100,000. "We're gonna take a trip sometime, but not right away. We haven't really had any time off since we got married."

"Unless you count Ocean City," he muttered.

She smiled. "Maybe we'll go to Florida, where we had our honeymoon. Or Las Vegas. In a couple of weeks we'll give a party for our friends. We'll invest it. It sure will come in handy with the colleges."

The four oldest all have jobs. They will keep them.

"Work makes good people," said Jean Tansill. CAPTION: Picture, $100,000 winner Jean Tansill with Bob Brumheller of McDonald's, by Vanessa Barnes Hillian