The ability of resting and activated rat bone marrow-derived mononuclear phagocytes (BMM phi) to bind monomeric rat, mouse, and human IgG was determined by means of flow cytometry. Rat IgG2b bound with high affinity (Kd approximately equal to 3 x 10(-9) M); binding was optimal at 4 degrees C and was only little affected by trypsin treatment. The other IgG bound with only low affinity (rat IgG2a, mouse and human IgG) or not at all to rat BMM phi (rat IgG1, rat IgG2c). The binding of rat IgG2b was not affected by the presence of a surplus of low-affinity binding IgG, and vice versa, indicating that high- and low-affinity IgG bind to different sites. Binding of high- and low-affinity IgG as well as expression of MHC class II molecules and of tumoricidal activity by BMM phi was markedly enhanced by rat interferon-gamma in low concentration (0.1 to 1.0 IU IFN-gamma/ml). On the other hand, heat-killed Corynebacterium parvum organisms, that were equally potent in triggering tumoricidal activity, neither enhanced the binding of IgG nor the expression of MHC class II molecules by BMM phi, suggesting that these abilities are not necessarily closely related phenomena.