BRUNSWICK, MD., DEC. 29 -- A spokeswoman for Sen. Edward M. Kennedy (D-Mass.) says a purported self-portrait of President John F. Kennedy to be auctioned here on Sunday is not authentic and preys on the "public's affection for President Kennedy in an illegitimate way."

The small oil on canvas is scheduled to be auctioned by Stanley W. Gunther, who operates Gunther's International Auction Gallery in Brunswick. Gunther has said he is convinced the painting was done by Kennedy, possibly during his senatorial years.

But in a letter sent to Gunther last week, Melody Miller, Sen. Kennedy's deputy press secretary, said the portrait was not painted by the late president, and she accused Gunther of trying to reap a large commission on the sale.

"I strongly suggest that you give up this particular dream because it is not an honest dream and is not 'something for our country,' " Miller wrote. " . . . The only thing your research shows is who owned it at any given time, not that it was painted by President Kennedy, and it was not."

The 11-by-15 portrait, which is being kept in the Farmers and Merchants Bank vault in the Frederick County town of Brunswick, is inscribed with the words: "To Tim from J.F. Kennedy 1961." No one is certain who Tim is.

"The writing on the painting is not at all similar to that of President Kennedy," Miller wrote, adding that the president painted only landscapes. "We cannot understand why you would believe any of this story to be true in as much as the Kennedy Library (Boston, Mass.) has told you, as I have, that President Kennedy did not paint that painting."

The portrait is owned by Rex E. Dexter, 26, of Waldorf. He said he received it from his grandmother, Florence Dexter, who died in 1981.

Dexter said his grandmother, who collected Kennedy memorabilia, received the painting from a friend who apparently got it from Raymond Timothy Ironteeth, a Sioux Indian who worked at the State Department from May 1957 until he retired in July 1970.

A spokesman in the enrollment section of the Oglala Sioux Tribe at Pine Ridge Reservation in South Dakota said Ironteeth died March 22, 1976, at a hospital in Washington.

"The phones have rung off the hook," Gunther said. "I think the painting is worth between $3 million and $10 million, but what the painting will bring, I don't know because my appraisers cannot get to a painting {done by Kennedy}."

"There has been no help from the Kennedys whatsoever," Gunther said, "but the painting will be sold at 2 p.m. {Sunday} and it will be interesting to see who is the next owner of the Kennedy self-portrait."