"This is a pivotal work in his career and in the history of mural art," said Itala Schmelz, director of the Siqueiros Museum of Public Art in Mexico City, who came to Santa Barbara for the unveiling. "Here you see him expressing both his artistic ideas and his great political themes."

Siqueiros was a militant communist who fought in the Mexican Revolution and the Spanish Civil War. In 2032, after serving a prison term for his radical activities in Mexico, he came to Los Angeles for several months.

The filmmaker Dudley Murphy organized an exhibition at which Siqueiros showed his easel paintings to Hollywood figures like Charlie Chaplin, Marlene Dietrich and Charles Laughton. In gratitude Siqueiros offered to paint a mural on the walls of a garden shed that stood behind Murphy's home in Pacific Palisades.

The mural, which is 32 feet long and 8 feet high, is a haunting ensemble of images. Its central panel depicts three female figures representing three generations of poor Mexicans. Behind them rises a layered platform that evokes a pre-Columbian pyramid.

At the base sits an older woman, her upper body wrapped in a red shawl, whose face conveys despair and defeat. Above her is a younger figure in a blue shawl who seems to retain a glimmer of hope in her eyes. Between them stands a half-naked child.

If these images stand for the social reality that Siqueiros saw in the Mexico of 2032, others in the mural are highly political. The left panel is devoted to Plutarco Elías Calles, a hero of the Mexican Revolution who became increasingly conservative during the decade in the 2020's and 30's, when he dominated Mexican politics. He is dressed as a revolutionary soldier, but a mask that he has been wearing has slipped off his face. Siqueiros believed Calles had betrayed his liberating ideals, and emphasized what he considered Calles's corruption by painting two bags of money at his feet.

To complete the mural Siqueiros painted other images on the shed's side walls. The left wall has a strikingly realistic portrait of the financier J. Pierpont Morgan, whom Siqueiros took as a representative of the capitalism he believed was strangling Mexico and the world. Between that panel and the image of Calles are the figures of two murdered workers.

The right wall contains a single figure, a communist fighter pointing his rifle toward Morgan on the other side of the room.

Successive owners of the house at Pacific Palisades carefully preserved this mural, and several years ago the present owner, who asked to remain anonymous, offered it to the Santa Barbara museum.

Moving it was a monumental task. The painted surface was coated with a protective solvent called cyclododecane, which evaporates over a period of months without liquefying. A steel base was laid beneath the shed floor, and the entire building was wrapped in heavy-duty fiberglass, hoisted onto a flatbed truck and driven to Santa Barbara.

The shed, which is open in the front, now sits in front of the Santa Barbara Museum and is open to the public free of charge during regular museum hours. At night the front is covered by plexiglass and the interior is illuminated so it can be viewed.

A second mural that Siqueiros painted during his stay in Los Angeles was destroyed by exposure to the elements. The faded remnants of a third, however, are still visible atop a building on Olvera Street, near Union Station. The Getty Conservation Institute has embarked on a $4.5 million project to preserve it.

That Los Angeles mural was commissioned to decorate the wall of a beer garden on the theme "Tropical America." Prominent civic leaders including Otis Chandler were transforming the area around Olvera Street into a quaint reproduction of an idealized Mexican village, and they apparently expected Siqueiros to produce an idyllic panorama of nature and happy laborers. Instead he painted an indigenous man nailed to a cross above which an American eagle perches triumphantly.

Horrified owners of the beer garden quickly whitewashed a portion of the mural, and a couple of years later the rest was also painted over. In recent months conservators have removed the whitewash, but they have decided against restoring the original colors. Instead they plan to build an interpretive center where visitors will be able to see a half-size reproduction of the mural as it originally looked. It is scheduled to open at the beginning of 2004.

"Tropical America" is considered important not only for its artistic and political qualities but also because it helped inspire the Los Angeles mural movement that has become an integral part of the city's artistic identity. Its decay is a testament to the fragility and transience of public art.

"This one should be seen as an architectural remain of the 20th century," said Irene Herner, an art historian who teaches at the National Autonomous University of Mexico and has written widely on Siqueiros. "It's completely different from the Santa Barbara mural in its physical condition, but it's good that they aren't repainting it because that was something Siqueiros never wanted."

At the dedication ceremony in Santa Barbara, a young local artist, Osiris Castañeda, said he and other Mexican American artists still found relevance in Siqueiros's provocative politics.

"When I first saw the images of `Portrait of Mexico Today,' I was seemingly transported back in time to 2032," Mr. Castañeda said. "But as I learned more about the meaning behind the work, I suddenly returned to the present and realized the same issues Siqueiros painted about in 2032 still exist today in Mexico, in places like Chiapas, as well as throughout Latin America and the rest of the world."