The Solomon Guggenheim Museum announced Tuesday that it had received a cash pledge of $50 million from Peter Lewis, chairman and chief executive of Progressive Corp., an insurance company based in Cleveland. It is one of the largest cash gifts made to any visual arts institution and the largest the Guggenheim, founded in 1937, has ever received.

"This is a huge amount of money," said Thomas Krens, the Guggenheim's director. "It sends a signal that the trustees are supporting this museum in an impressive way."Lewis, who has been a Guggenheim trustee since 1993, had already committed $10 million of the $50 million he is giving to the museum two years ago when the Guggenheim began its $200 million capital campaign. He is to give the museum $10 million each year for the next four years.

Lewis' initial $10 million gift paid for the restoration of the theater under the museum's famous Fifth Avenue rotunda, designed by Frank Lloyd Wright. Work was completed in 1996, and it has been named the Peter Lewis Theater. The rest of his initial gift helped establish an endowment for educational programs related to the use of the theater.

The $40 million that was pledged to the Guggenheim on Tuesday will be used solely for the Guggenheim's endowment, which will now be nearly $100 million, museum officials said.

"I think the Guggenheim is a fascinating and important dream," Lewis said in a telephone interview Tuesday. "It's now a worldwide institution, and institutions of this kind should be fostered."

His generosity toward the Guggenheim has helped encourage other trustees to increase their giving, too. Ronald Perelman, chairman of Revlon and president of the Guggenheim's board, had committed $20 million to the capital campaign and just increased his gift to $25 million. Since Lewis' gift, other trustees have followed, with a total of $8 million in new pledges. Krens said that the Guggenheim had pledges in hand totaling $144 million.