WHILE DRIVING through Mexico in the late 1950s, Aaron Siskind noticed for the first time how the graffiti there reminded him of paintings by his friend Franz Kline.

Two decades later, Siskind's germ of an idea reached fruition in his series "Homage to Franz Kline" -- more than a hundred photographs, 48 of which are displayed now at the Museum of American Art.

But call them photographs only in the technical sense of images made through a lens; there the similarity ends. They challenge our expectations of photographs while confounding us with symbolism.

Franz Kline's abstract expressionist paintings were energetic, rough brushstrokes -- usually black, balanced on a field of white. Siskind closed in on graffiti until they appeared to be abstract expressions. In Jalapa, Mexico, in Lima and in Rome, he painstakingly composed his photos through the lens, achieving the same balanced composition one finds in Kline.

Like Kline, Siskind chose black and white; he printed dark to emphasize shapes and textures. And the graffiti he photographed were obviously done with cheap paints and housepainter brushes -- curiously, the same sorts of materials Kline preferred.

Close up, the writing no longer spells out Spanish or Italian, but universal symbols -- an "X" boxed in, a dollar sign, an alphabet forming on the nubby surface of Roman walls. Or the writing/painting is more abstract -- slaps, scratches, scrapes and sprays, far from Bruce Davidson's photos of New York subway cars.

As Siskind, now 82, frequently comments on the human condition in his work, we can assign meaning to his cloudlike creatures slipping from primordial ooze, and paint dripping like blood.

His camera focused less on content than on the feeling and energy that went into making the graffiti -- closeups of the rush of the brush. Yet among these asymmetrical ink-blot tests, occasionally a sign post reminds us that we're looking at walls. Someone has etched out a horse, or slapped the word "NO" to obscure other powerful words. And "Jose 6/5/76" has signed his name. HOMAGE TO FRANZ KLINE: PHOTOGRAPHS BY AARON SISKIND -- At the Museum of American Art through March 3, 1985.