The heirs of the late art dealer Ileana Sonnabend have arranged to sell two groups of major artworks to pay taxes on her estate, Sonnabend’s adopted son, Antonio Homem, said yesterday. According to the estate lawyer, Ralph Lerner, Sonnabend’s collection, which is still in the process of being appraised, may be worth as much as $600 million. Of the two groupings of works being sold, one is several pieces by Andy Warhol; Mr. Homem declined to name the buyer because the sale has not been finalized. The other, he said, comprises mostly Pop art works from the 1960s. Again, he declined to name the buyer, in this case because of a confidentiality agreement. But he said he expected those works to be on public view in the near future.

Rumors have swirled recently that Sonnabend’s collection might come up for auction. Sonnabend died in October, so her heirs are required to pay estate taxes by July. Mr. Lerner confirmed that the estate has been in touch with the two major auction houses but said: “We are not having a big auction in May.”

Along with her first husband, Leo Castelli, Sonnabend helped launch the careers of some of the most important American artists of the late 20th century, including Warhol, Jasper Johns, Robert Rauschenberg, and Roy Lichtenstein. In 1962, after divorcing Castelli and marrying Michael Sonnabend, she opened her own gallery in Paris, in order to introduce these artists, and others such as Jim Dine, Claes Oldenburg, and James Rosenquist, to Europe.

In 1971, she opened a gallery in SoHo, with a debut show of the British artists Gilbert and George. In 1972, the artist Vito Acconci did a show at the gallery called “Seedbed,” which consisted of him masturbating, out of sight under a wooden ramp, but with his groans and muttered words piped through loudspeakers.

From the beginning, Sonnabend collected works by the artists she represented. “She was always much more a collector than Leo,” Calvin Tomkins, who profiled Sonnabend in the New Yorker in 2000, said in an interview. “She made a point of buying the pieces that didn’t sell, which are often the most difficult pieces, and the ones that turn out to be the most important.”

The collection has never been cataloged, and was never even fully inventoried until Sonnabend’s death. “We never counted,” Mr. Homem said.

Selections from the collection have been exhibited in museums, and many works are on long-term loan to museums around the world. An early work by Mr. Rauschenberg, “Canyon,” whose major element is a stuffed eagle, is at the Los Angeles County Museum of Art, and numerous pieces, including works by Warhol, are on loan to the Baltimore Museum of Art.

Sonnabend did not express much interest in donating works to museums. She told Mr. Tomkins in 2000: “I am not so enchanted with museums. Things change in museums, directors change. I would much rather sell to collectors, who appreciate what they’ve got.”

If works from her collection did come up at auction, Mr. Tomkins predicted that her name would attract great interest. “Provenance is always very important, particularly in auction sales, and I think everybody knew that her reputation was of having this marvelous eye and great courage in what she bought,” he said.