In vitro granulomas are induced by culturing murine spleen cells with artificial microparticles, dextran beads. In the presence of 0.5 microgram/mL 8-methoxypsoralen, UVA radiation (0.2-2.0 J/cm2) suppressed granuloma formation in a UVA dose-dependent manner. The doses of PUVA did not affect the cell viability as assessed by trypan blue exclusion. The time course of granuloma formation in 0.5 J/cm2 PUVA-treated cells was similar to that of normal spleen cells, with a maximum granuloma index at day 3 of culture, although a 49-63% suppression of granuloma formation was observed. PUVA-treated, nonadherent cells produced the same granuloma index when cultured with normal adherent cells. In contrast, a smaller granuloma index was observed in PUVA-treated adherent cells even when they were cultured with normal nonadherent cells. These data suggest that PUVA alters macrophages, resulting in the suppression of granuloma formation in vitro.