We have studied the effects of Fc receptor triggering on the free cytosolic Ca2+ concentration, [Ca2+]i, in U937. These cells express two types of IgG Fc receptors, Fc gamma RI and Fc gamma RII. Binding of several anti-Fc gamma RI and anti-Fc gamma RII mouse monoclonal antibodies (MoAb) to Quin2- or Indo-1-loaded U937 cells had no direct effect on [Ca2+]i. After addition of a bridging anti-mouse Ig antibody however, transient increases in [Ca2+]i were observed for both types of Fc gamma R. One of the anti-Fc gamma RII MoAb, CIKM5, was exceptional in that it could induce Ca2+ increases in U937 cells by itself. Studies with F(ab')2 fragments of CIKM5 revealed that this MoAb simultaneously binds to Fc gamma RII, via both its Fab and Fc fragments, which might induce cross-linking of two Fc gamma RII molecules. One anti-Fc gamma RI MoAb, 197, a mouse (m)IgG2a antibody directed to an epitope outside the IgG-binding region, remarkably also caused an immediate increase in [Ca2+]i, but only when added to U937 precultured with gamma interferon (IFN-gamma). Fc gamma RI can bind monomeric human IgG as well as mIgG2a, and cross-linking of cytophilic Ig induced an increase in [Ca2+]i. Our results show that [Ca2+]i increases can be induced only after cross-linking of Fc gamma R, either via anti-Fc gamma R MoAb or via Fc-FcR interactions. Furthermore, we show that Fc gamma R cross-linking results in activation of the Ca2+/phosphatidylinositol (PI) signal transduction pathway.