"Once, in college, I played in 'The Taming of the Shrew' in modern costume," that old trouper Ronald Reagan recalled yesterday with a faraway, Shakespearean look in his eyes. "You can imagine how long ago that was when I tell you that my costume was plus-four knickers."

Receiving a group of fellow Shakespeare fans at the White House, the president recalled a critic's line about an actor who played King Lear "as though someone else had played the ace." In contrast, for a select audience of 250 people who flocked to the East Room as part of a $1,000-a-ticket package, Ronald Reagan played the role of president as though the script had been written by a Shakespearean scholar.

The occasion was a double celebration--not only the 50th anniversary of the opening of the Folger Shakespeare Library, but the 418th birthday of William Shakespeare, both of which are today. In a single evening, the library raised $250,000 with a benefit gala that included cocktails and a shake of the presidential hand at the White House. Later there was a concert of Elizabethan music, a 20-minute performance of moments from Shakespeare by Ian McKellan, an elegant, white-tie dinner by candlelight in the Great Hall and Reading Room of the Folger Library and enrollment in the "Order of the Folger Shakespeare Library," a new honorary society whose members received a gold-colored medal on a white ribbon.

In the library's Great Hall, which is modeled on the great hall of an Elizabethan castle and has banners bearing Elizabethan coats of arms hanging from the ceiling, 250 of the medals sat on a side table at the beginning of the evening, waiting to be presented to the $1,000 donors. A line of tables stretching across the hall and through the adjoining Reading Room was decorated with large topiary plants in the forms of geese, boars, lions, unicorns and other heraldic animals.

Guests at the gala benefit included a good representation of Washington society (Gwen Cafritz made it to the White House but not the library) as well as members of the Reagan inner circle (Edwin Meese III, Michael K. Deaver, James A. Baker III, William P. Clark, Caspar W. Weinberger), Sens. John G. Tower (R-Tex.) and Paul Laxalt (R-Nev.) and "representatives from half the major corporations in America," according to a library spokesman. Expenses of the party were underwritten by Mr. and Mrs. Richard Manoogian.

Others included Ambassador and Lady Henderson of Great Britain, former ambassador and Lady Ramsbotham, Viscount De L'Isle and Mrs. George Angus Garrett, who said that the party was "the most beautiful in the last several decades."

There was also an Elizabethan flavor (though a less intense one) at the White House, where many of the guests had visited before. "I have to see whether they've changed any of the pictures," said one woman who had not previously been to the Reagan White House, wandering through the rooms and examining the walls.

"Where's the marine?" asked a White House veteran, strolling into the diplomatic reception area after being cleared through the metal detector at the entrance. Instead of the lone marine playing a harp who usually greets guests entering there, three bearded men in tuxedos--the members of the Folger Consort--were playing Elizabethan music on lute, recorder and viola da gamba. As the library's house orchestra, they entertained not only at the White House, but in the library's Elizabethan theater and at the dinner, where they climaxed the evening's festivities by playing arrangements of "God Save the Queen" and "The Star-Spangled Banner" on their Renaissance instruments.

At the reception in the White House, strolling minstrel David Perry wandered among the cocktail-sippers, singing songs such as John Dowland's "I Saw My Lady Weep" and accompanying himself on a lute. "It's a difficult room to play," reflected Perry, who had trouble finding an audience among the nosh-and-slosh crowd around the bars and buffet tables.

In the grand foyer, where the Marine Band usually holds forth at receptions, the 12-member New York Renaissance Band played lutes, sackbuts, viols, recorders and percussion. Like the Marines, the New Yorkers were in uniform, but their uniforms were colorful Renaissance costume. "In delay there lies no plenty," sang a tenor. "Then come kiss me, sweet and 20. Youth's a stuff will not endure." Not the sort of thing one usually hears at a White House reception.

Ronald Reagan's induction into the Order of the Folger Shakespeare Library was handled by the library's director, O.B. Hardison, who read a scroll citing the president for his "meritorious contribution to the arts and culture in America." The president also was given the pen with which the citation had been signed, a special Folger Shakespeare Library pen inscribed with a quotation from "King Lear": "Keep thy pen from lender's books."

Reagan recalled that "someone said" that "we could find the answer to all the complexities and troubles of the world . . . just by opening the covers of books."

Calling the Folger Library a "priceless treasure," he expressed the hope that the library would have another 50 years "of bringing insight and enlightenment. . . .

"All Americans can be proud that the finest collection of Shakespeare is on this side of the Atlantic," Reagan said. "It belongs to all mankind, but it's in our country."

He took a philosophical attitude to the news that the Folger medal was not made of real gold.

"If it was pure gold," he said, "David Stockman would have gotten it by now."