The White House has accepted what it calls "the most valuable gift in White House history."

It is a formal portrait of Gen. George Washington which, although a replica, was recently appraised by a dealer in New York at $675,000. The late husband of the donor purchased it at auction -- for $41,600 -- in 1965. Because Mrs. Lansdell K. Christie gave it to the nation, her gift it tax deductible.

The portrait is the work of Charles Willson Peale (1741-1827), the famous Philadelphian for whom the father of our country sat no less than seven times. p

This picture is a product -- although not the first -- of the second sitting, which took place in May, 1776, when Washington was still a revolutionary general. The original painting Peale did from life is now installed in Brooklyn. But, as was often done in those pre-photographic days, he then produced two copies.

The first, a miniature, was done for Martha Washington; the second, which is full-size, was bought by a "French gentleman" who took it off to Europe. It is this work that now hangs above the mantle in the Oval Office. It has been on loan there since 1971.

"Landsdell K. Christie," says White House curator Clement Conger, "was one of the great collectors of early American furniture. I knew him in the war. An Army aviator, he used to fly the milk run between Dakar and North Africa, and he was clever enough to notice that when he passed over one portion of Liberia, the needles of his compass seemed to go berserk. He pinpointed the place, and when the war was over founded the Liberian Mining Co. There was iron there below." The Peale replica had just been rediscovered in a house in Ireland when Christie purchased it at Christie's (no relation) in July, 1965.

In the 15 years since then, according to the appraisal of New York's Robert Graham, a well-regarded specialist, the painting's value has appreciated more than 1500 percent.

Under U.S. tax laws, the donation is considered a $675,000 gift; a sum that may offset half the donor's income for as many as five years. "I am sure that Mrs. Christie will use all of those five years," said Conger. "She is not nearly as wealthy as her husband; in fact, I doubt that she will benefit from the whole deduction."

The Peale was accepted by Rosalynn Carter and the other members of the Committee for the Preservation of the White House at their semi-annual meeting Oct. 26. It was not their only acquisition.

The privately supported committee has purchased "A Day at the Seashore," a California genre scene by the German-born William Hahn (1829-1887); it cost $49,500. Another oil painting, "Spring in the Vallery, Near Chester, Vermont," painted circa 1921 by Willard Leroy Metcalf (1858-1925), was purchased for $36,000. Both these works, in Conger's phrase, are "available for donation," meaning that the committee hopes some donor will reimburse the committee for the purchases.

The committee also bought a Duncan Phyfe mahogany table for $9000, and two Regency English lanterns -- one for $2200, the other for $1050. In addition, it received various antique gifts, a swatch of 1887 White House wallpaper, an 1820 silver coffee service, and a Meissen cup that also bears a Peale likeness of Washington.

In an 1830 guidebook, also given to the White House, the author, Samuel Lorenzo Knapp, complains that "the ornaments" there "are sparse and not of high order. . . . This palace belongs to the people, and should be adorned with the best specimens of the fine arts the country can produce." Conger has been working since the Kennedy Adminstration to accomplish that.

His committee recently established The White House Preservation Fund, which is seeking $25 million, the interest from which would go for paintings, rugs and other White House ornaments. Two members of the committee, Walter Shorenstein of San Francisco and Richard Manoogian of Detroit already have contributed $100,000, each; five other committee members have given $25,000 apiece.