To investigate the basic mechanisms of granuloma formation, in vitro granulomas were induced by culturing murine spleen cells in the presence of artificial microparticles. Large granulomas developed around dextran beads. The lesions were inducible by spleen cells from either normal mice or athymic nude mice. Minimal inflammation was produced around latex beads. The histologic features and time kinetics of granulomas in vitro. Culture supernatants of dextran induced granulomas contained high levels of interleukin-1 (IL-1) activity but not interleukin-2 (IL-2) or interleukin-4 (IL-4) activity. IL-1 activity was correlated with granuloma size. Additionally, granulomas were produced by culturing spleen cells in the presence of agarose beads coupled to recombinant IL-1 or recombinant tumor necrosis factor-alpha (TNF-alpha). Granulomatous lesions also were induced by macrophage-enriched populations in the presence of monokine-coupled beads. Adherent macrophages, but not nonadherent cells, were required for induction of granulomas in vitro. In contrast, very small lesions were seen when spleen cells or adherent cells were cultured in the presence of beads coupled to recombinant IL-2 or recombinant interferon-gamma (IFN-gamma). These results suggest that macrophages and monokines such as IL-1 and TNF-alpha play an essential role in granuloma formation in vitro.