WASHINGTON (AP) _ It was 50 years ago that the alchemists at Du Pont turned coal tar into nylon and unwittingly sparked a social revolution that put sheer stockings on the legs of millions of women who couldn't afford the luxury of silk.

Nylon was first used for toothbrush bristles, then fishing lines and surgical sutures, but it was nylon stockings that women fell in love with.

''It was the best thing that ever happened to a lady,'' said Isabel Davis, 64, of Los Angeles, who fondly remembers getting her first pair of nylons when she was graduated from junior high school. She was 14.

''I really felt like a lady,'' she said. ''I have four sisters and we all loved them. I still wear them, with rhinestones and flowers and the old- fashioned kind with seams in the back.''

A display of nylons created a sensation at the New York World's Fair in 1939. When they first went on sale in New York City stores on May 15, 1940, women bought up 4 million pairs in a few hours. A depression suddenly hit the Japanese silk market.

''They were marvelous,'' said Grace Lyons of suburban Washington, who grew up in the Bronx, N.Y. ''They were like iron. They'd last for a year.''

Then, just as women got accustomed to wearing nylons, the stockings vanished from store shelves during World War II. The fabric was melted down to make parachutes and aircraft tires. Movie pin-up girl Betty Grable peeled the nylons off her shapely legs at a war bond rally, and they were auctioned for $40,000.

Teen-agers unable to find nylons on the black market painted black seams down the back of their legs to impress their boyfriends. Young women suffering the drab austerity of wartime London discovered that friendly American GIs were willing suppliers of nylons, obtainable at the Post Exchanges when they weren't available elsewhere.

A mob of 10,000 shoppers descended on Market Street in San Francisco for the first postwar sale of nylons in 1945. A department store window was shattered, several women fainted and the sale was canceled. Hollywood celebrated the return of nylons by hoisting actress Marie Wilson aloft on a crane to examine a 35-foot replica of her nylon-sheathed leg.

Londoner Ivy Bull, 75, recalls that she could only wear cotton lisle stockings until friends returned from the United States in 1946 bringing her a gift of her first pair of nylons.

''You didn't wear them every day,'' said Mrs. Bull. ''They were just a couple of pairs and you just took care of them. You wore them if you were going anywhere special. With your best outfit, that was the idea.''

Nylons would get runs - the British call them ''ladders'' - if a thread was caught on a jagged fingernail. It's common knowledge that a dab of fingernail polish will stop a run from getting any worse.

Mrs. Bull would take her stockings to a repair shop and ''they would clean up the ladders for you.''

Mary Pennybacker, a native Londoner who lives in Washington, remembers her mother hanging laddered nylons from the mantel as Christmas stockings rather than tossing them out.

Nylons also are useful to bank robbers, who sometimes wear them over their faces as a disguise. And nylon has become a fantastic money machine for Du Pont. Corporate officials say sales of the synthetic fiber have totaled more than $10 billion since Du Pont's first nylon factory began operations in Seaford, Del., a half-century ago.

None of the team of Du Pont research scientists who invented nylon in the 1930s could have envisioned that an estimated 8 billion pounds would be manufactured worldwide 50 years later, for use in thousands of items from clothing and carpets to lacrosse sticks and circus tents.

After years of experimenting with man-made polymers, or bead-like strings of molecules, Du Pont chemists under Wallace H. Carothers, a wizard hired from Harvard University, produced a tough, durable, flexible fiber using chemical building blocks derived from coal tar, air and water.

But ''nobody said, 'Eureka 3/8''' remembers Julian Hill, 83, a Carothers associate now comfortably retired near Du Pont headquarters at Wilmington, Del. ''I suppose we had hopes, but we were doing basic scientific research and it just happened that this led to something that turned out to be important.''

It was Hill who discovered that a molten polymer could be drawn into filaments, cooled and stretched to form very strong fibers - an early breakthrough toward the invention of nylon. Last week, at a Washington luncheon celebrating the 50th anniversary of nylon and Teflon, another Du Pont invention, Hill sat chuckling as models wearing nylon bikinis pranced down a runway.

Carothers, who suffered fits of severe depression and was convinced he was a failure, committed suicide in a Philadelphia hotel room three weeks after the nylon patent application was filed in 1937, and six months before the birth of his only child.

When the invention was announced in New York in October 1938, says Du Pont chairman Richard E. Heckert, the story was buried on Page 24 of The New York Times and almost ignored by The Wall Street Journal. Du Pont stock fell by a point and a half on Wall Street.

But when Du Pont research vice president Charles M.A. Stine made the announcement before a national women's conference, the audience applauded loudly when he said nylon could be used for run-resistant stockings.

The first nylons were advertised for a top price of $1.35 a pair, which was a lot of money in 1940 but a lot cheaper than silk stockings, which cost two or three times as much.

Silk stockings were extremely delicate and easily torn, didn't keep their shape and became baggy at the ankles from body heat. But their owners, lucky or affluent enough to afford them, carefully wrapped them in tissue paper as treasured symbols of glamor and elegance.

Before nylons, most working women had to settle for cotton or wool hosiery.

Today, according to the National Association of Hosiery Manufacturers, American women spend about $3.5 billion annually on sheer hosiery, 99 percent of it containing nylon.

Those black seams down the back disappeared by the early 1950s, when textile mills discovered how to knit hosiery yarn in cylindrical tubes.

The advent of stretch nylon in the late 1950s led to the introduction of waist-high pantyhose, which allowed women to throw away their garters and belts and join the miniskirt revolution of the 1960s. Pantyhose now account for 70 percent of hosiery sales today.

There once was speculation, when war was declared against Japan and silk supplies were halted, that nylon was an acronym that stood for ''Nuts, You Lousy Old Nipponese.'' Actually, nylon is a meaningless word, coined by Du Pont simply because it is easy to spell and pronounce.