A once-swank hotel that was brothel, bar and headquarters for Chicago crime czar Al Capone will be restored as a museum and center celebrating international womanhood.

The Sunbow Foundation, a nonprofit activist women's organization, is planning the museum and center in the Lexington Hotel, which was once a Gilded Age showplace later equipped with secret staircases and tunnels for Capone. The hotel is now a gutted derelict standing amid other abandoned, vandalized buildings.

On Wednesday, the City Council designated the Lexington a historic landmark, both because of Capone and because of the building.

"We're looking at the structure itself," cautioned Carol White, an official of the Chicago Commission on Historical and Architectural Landmarks, when asked about the Capone connection. "It had hollow tile fireproofing and other advances."

The 400-room hotel opened in 1892 with numerous innovations for the time, including electric lights, telephones in the elevators and a vault for guests' valuables. The National Trust for Historic Preservation designated the building a historic landmark in December 1983. Sunbow bought it for $500,000 two years ago.

Ruth Ann Fowler, an official at Sunbow, said the idea of housing a museum of women's achievements in the building struck Sunbow as especially appropriate, since there had been a women's building at the 1893 Columbian Exposition, which was a key element in the resurrection of Chicago after the Great Fire of 1871. "We thought, 'Hey, in 1893 there was a women's building . . . and what a tie-in to the 1992 fair."

Sunbow, which has federal grants to train women in the construction trades, intends to reopen the hotel as a rental apartment building in time for the planned 1992 Chicago World's Fair.

But the interest today centers on the Capone connection. An unopened vault reportedly has been found in the hotel basement. "Maybe it's full of Capone money," one official speculated. "Wouldn't that be something?"