The most important collection of Andrew Wyeth paintings outside of the Brandywine River Museum in Chadds Ford, Pa., Wyeth's hometown, has been sold to an unnamed buyer for an undisclosed price likely to be more than $25 million. This is the second major Wyeth collection sold in a year.

Arthur and Holly Magill, whose family fortune came from Her Majesty Industries, manufacturer of children's clothing, yesterday announced the sale of 26 paintings, which -- like the collection sold last year -- had been on extended loan to the Greenville County Museum of Art in South Carolina. The collection sold yesterday put the small museum on the map after it was purchased in 1979 from movie producer Joseph E. Levine for $3.1 million and installed in a special gallery.

Among the seven temperas on wood and 19 watercolors sold yesterday are some of Wyeth's best-known images, among them the temperas: "The Quaker," two coats hanging from a fireplace mantle; "Weather Side," a painting of Christina Olsen's house; and "Rum Runner," a man sitting on a skiff looking at the sea off the coast of Maine.

Arthur Magill said yesterday that he could not reveal either the sale price or the purchaser's name because he and his wife had signed confidentiality agreements.

The Magills also announced, simultaneously, their pledge to donate, through the Arthur and Holly Magill Foundation, $6 million to the museum, of which they have been primary supporters. In 1973, Arthur Magill gave $750,000 to launch the building fund.

This $6 million gift is to be used by the museum to support art acquisitions, exhibitions and educational programming, according to a museum release. Though it is the largest single-donor contribution to an arts institution in South Carolina history, it falls far short of the total value of the Wyeths sold yesterday.

Last July, the Magills sold another group of 228 Wyeth drawings, watercolors and one tempera painting, known as the Olsen collection, to Leonard Andrews, owner of the much-ballyhooed Helga collection. At the time Magill said there would be no further sales, and that the 26 paintings sold yesterday would remain at the museum.

Andrews, who called his Wyeths a "national treasure," subsequently sold both the Helga and Olsen collections to a Japanese buyer for more than $40 million last year.

"We're very sorry to see the Wyeths go," said Paula Hysinger, spokeswoman for the museum. "But we're also delighted to have the $6 million for acquisitions, exhibitions and educational support.

"Our permanent collection goal has been to present Southern-related art, either created in the South or by Southern artists, and to survey American art history through Southern art, from the Colonial period to the present," she said.

"I'm very pleased with what the Wyeth collection has meant to Greenville," Magill told a reporter. "It's brought national attention to a rather obscure museum, and I think with the money we're donating, the future for the museum looks very bright."

In 1979, Magill's purchase of the collection from Levine caused the resignation of Jack A. Morris Jr., then the museum's director, who was accused of accepting unethical commissions on the sale of art works to Magill.