The Smithsonian Institution's plans to build two largely underground museums onthe Mall -- one for African art and another for Eastern art -- have been opposed unanimously by the Joint Committee on Landmarks as "alien architectural elements" that would ruin the Mall's only historic buildings, the 1855 Smithsonian "Castle" and the 1881 Arts and Industries Building.

The criticism by the Joint Committee, the city's 13-member historic preservation advisory board, comes after two months of reviewing comment on the proposed museum designs from the Department of Interior, the Environmental Protection Agency, the Sierra Club, and the prestigious Committee of 100 on the Federal City. Most of the reaction was negative.

More than 90 percent of the space in the two museums would be underground, with two 2-story pavilions above ground in the four-acre quadrangle, or South Yard, behind the Smithsonian Castle. A Victorian garden and a 140-car parking lot for Smithsonian employes now occupy the quadrangle, which is surrounded by the Freer Gallery of Art, the Castle, the Arts and Industries Building and Independence Avenue.

The cost of the $50 million project would be split evenly between the federal government and the Smithsonian, through donations and funds raised by its museums and projects.

The Joint Committee has opposed the proposed pavilions, buried light wells, walls and underground parking lot ramps as disruptive and stylistically out of place beside the 19th century buildings.

Lawrence Taylor, spokesman for the Smithsonian, said this week that "there is a difference of opinion over the aesthetic effect" the two museums would have on the Castle and the Mall. But he said the Smithsonian is continuing to make changes in the museums' design and hopes that, despite the criticism, the plan will be approved at the January meeting of the National Capital Planning Commission.

The Smithsonian already has reduced the proposed length of the two pavilions and eliminated the light wells -- large, glass-walled pits designed to admit light into the museums' underground levels.

NCPC, Washington's federal planning agency, which has veto power over the museum proposals, is advised by the Joint Committee on Landmarks and by Washington's major arbiter of building style and aesthetics, the Fine Arts Commission.

Fine Arts Already has given general approval to the concept of the two museums at its meeting last April, but also expressed reservations about the light wells, the size of the pavilions, and a garden that would replace the present Victorian garden and be planted on top of the underground museum building.

The Joint Committee, however, was scathing in its opposition to the two museums, and said in its November report to NCPC that "the Committee finds this proposal to be so adverse that it recommends reevaluation of the entire concept" of building in the quadrangle.

"It seems particularly inappropriate to remove the Museum of African Art from its present inadequate and cramped though historically rich quarters in the Frederick Douglass House on Capitol Hill to this anonymous and meaningless location in the garden of the Smithsonian Castle," the Joint Committee report said.

It urged, as have several people such as the Sierra Club and the Committee of 100 -- Washington's first planning agency founded in 1923 -- that a new Museum of African Art should go near the Gallery Place Metro stop, close to the National Protrait Gallery and the Smithsonian's newly named National Museum of American Art (formerly National Collection of Fine Arts). The District's new art center soon will open nearby in the old Lansburgs department store, and many private art galleries are moving into the area.

"At this time neither American art nor African art is represented on the Mall," the Committee of 100 wrote in commenting on the project. "It is difficult to believe that the Museum of African Art can have a 'presence' on the Mall if it is located in a far corner of the quadrangle and is mostly underground."

The Interior Department has urged that the height of the pavilions be lowered, and the Environmental Protection Agency has objected to the extensive underground parking facility proposed for tourists and Smithsonian staff. The EPA contends that the 350-space parking lot would encourage visitors to drive to the Mall, already one of Washington's worst areas of air pollution, although the museum would be almost directly adjacent to Metro's Smithsonian station.

While the Joint Committee objected to the two-story pavilions and the walls surrounding them, "visually isolating the Smithsonian Building from the city," it praised the proposed elimination of the surface parking lot and the underground museum space for the Freer.

The Freer, which houses one of the world's great collections of Oriental art, would become the nucleus of the new center for Eastern Art. Japan already has promised a $1 million gift to help build the center. The Freer is now too small to house visiting exhibitions.