IN the wake of a long revolution against dictatorship, Mexican artists vowed in the 1920s to create works that would instruct and enrich the masses. They even signed a manifesto proclaiming, "We repudiate so-called easel painting and every kind of art favored by ultra-intellectual circles." Out of this mood came the great murals of modern Mexico, especially the monumental works of Diego Rivera, José Clemente Orozco and David Alfaro Siqueiros.

But the mood also spawned a lesser-known burst of creativity — an enormous production of prints for 30 years. Unlike paintings that would likely be savored by rich families in their homes, the multiples of these woodcuts, linoleum cuts and lithographs could reach many people.

This wonderful side of Mexican art is celebrated in "Mexico and Modern Printmaking: A Revolution in the Graphic Arts, 1920-1950," which opened at the Philadelphia Museum of Art on Oct. 21. Unlike the companion "Tesoros" show, the Mexican prints are not scheduled to go on to Los Angeles. After the exhibition closes in mid-January, it travels to the Frist Center for the Visual Arts in Nashville, the Phoenix Art Museum, and the McNay Art Museum in San Antonio.

The show will surely serve as a revelation to many visitors. Not only does it make clear that muralists Rivera, Orozco and Siqueiros — described as "los tres grandes" in the catalog — and the other titan of 20th century Mexican art, Rufino Tamayo, were master printmakers. It also introduces a bevy of other fine Mexican artists — especially Leopoldo Mendéz — who were masters as well.

"Mendéz is one of the hidden figures in the whole movement," says John Ittmann of the Philadelphia Museum of Art. "He deserves a place, certainly as a printmaker, alongside los tres grandes and Tamayo." Ittmann is co-curator of the exhibition with Lyle W. Williams of the McNay Museum.

Mendéz, a member of the Mexican Communist Party, helped found the Taller de Gráfica Popular (the Print Workshop for the People) in 1937. Its manifesto promised that the artists would try to foster "the progressive and democratic interests of the Mexican people, especially in the fight against fascist reaction." The workshop's prints dealt mainly with Mexican tradition, social and political causes and glorification of the Mexican Revolution of 1910-20.

One of the most impressive prints by Mendéz, "Posada in His Workshop" (a 1953 linocut), pays homage to José Guadalupe Posada, a critic of the dictatorial regime of Porfirio Díaz around the turn of the century. The print shows an angry Posada with knife in hand as he works on a woodblock depicting a brutal scene outside the window of his print shop. Posada is watching troops beat and forcibly conscript peasants into the army.

The Philadelphia show devotes a good deal of space to the work of Rivera, Orozco, Tamayo, and Siqueiros. All spent time in the U.S. during this era, creating murals, painting canvases and learning print techniques. Tamayo even worked for President Franklin D. Roosevelt's Works Progress Administration Federal Arts Project during the mid-1930s.

Rivera produced only 13 prints in his lifetime, and almost all are on display in the show. Rivera was encouraged to try his hand at lithography by the Weyhe Gallery of New York, one of the few American dealers interested in prints and Mexican art in those years. The gallery's director, Carl Zigrosser, later became curator of prints at the Philadelphia museum, and his personal collection, purchased after his death in 1975, became the nucleus of the museum's strong collection of modern Mexican prints.

Rivera's murals were already famous in the early 1930s when he was persuaded that prints might please collectors who could not travel to Mexico to see the enormous frescoes.

His most familiar print, "Zapata" (1932), is a lithograph that copies a detail of one of his murals in the Palace of Cortés in Cuernavaca. It is an idealized portrait of the revolutionary hero Emiliano Zapata, who was assassinated in 1919. Other prints such as "Open Air School" (1932) were derived from Rivera's murals on the walls of the Ministry of Public Education in Mexico City.

But Rivera did more than refashion his murals. Works unrelated to the murals include the lithograph "Nude With Beads," a 1930 portrait of his wife, Frida Kahlo.

Orozco also made prints from his murals, but they amounted to a small portion of his work, for he was far more prolific than Rivera as a printmaker. Ittmann, writing in the catalog, says that Orozco found that lithographs and etchings were "ideal modes of expression for the biting humor and dark pathos of his grand artistic vision." When he lived in New York from 1927 to 1934, he supported himself mainly by printmaking.

Two of his most biting works, however, were lithographs he made in 1935 after returning to Mexico City: "Generals" and "The Masses." In the first, he mocks the pretensions of Mexicans who swagger and try to pass themselves off as revolutionary heroes. In the second, he mocks the screaming mouths of the foolish masses for whom the revolution was supposedly fought.

Many Mexican printmakers liked to work in wood because it was cheap, available, and the traditional material of such earlier artists as Posada. The exhibition displays a few of the original blocks, including the one for Tamayo's "Head II" (later retitled "Grief"). Tamayo, who worked on this in New York in the mid-1920s, carved the wood with a chisel and a penknife. His cuts seemed to create a crude mishmash of wood. Yet the result on paper was a haunting and subtle portrait.

Mexican printmaking during these years was heavily infused with left-wing politics. Yet there was continual bickering among supposed allies. Méndez, Rivera and Siqueiros were all communists but followed different byways of the movement.

Méndez satirized the other two mercilessly in his 1932 wood engraving "God and the Four Evangelists," which he also called "Fool's Concert." Resembling rag dolls, a foolish Rivera beats an Aztec drum while Siqueiros, sporting a crown, strums a sickle. Méndez even published the print as part of a fake flier advertising a radio concert by patients in an insane asylum.