The effect of murine IgG hybridoma antibodies directed against leukocyte antigens on the Fc receptor function of human cells was studied. For this purpose, the specific binding of 125I-labeled monomeric human IgG1 to a macrophage-like cell-line (U-937) was quantitated before and after incubation in the presence of murine monoclonal hybridoma antibodies. Four monoclonal hybridoma antibodies (A1G3, 23D6, 4F2, and 3A 10), each of which binds to different antigens on the surface of U-937 cells, rapidly and potently inhibited the specific binding of labeled IgG1 to these cells. Inasmuch as inhibition was mediated only by IgG antibodies with an intact Fc fragment and antibody activity against surface antigens found on U-937, inhibition appears to have resulted from the formation of a three-component complex composed of antibody bound by its Fab portion to antigen and by its Fc fragment to a Fc receptor. Equilibrium binding studies performed on treated cells confirmed that reduced Fc receptor-mediated binding was due to a reduction in the number of available receptors. Binding studies employing double isotope labeling methods demonstrated that about 0.5 to 1.0 Fc receptor was blocked for each molecule of intact antibody bound to a U-937 cell. Using several techniques, it was shown that most of the monoclonal antibody bound to cells and the Fc receptors blocked by antibody remained on the cell surface despite incubation at 37 degrees C for 3 hr. Thus, the loss of receptor function observed in these experiments was almost exclusively due to reversible receptor blockade rather than receptor internalization or degradation. The antibodies identified in these studies also markedly inhibited Fc receptors on one other human cell line (HL-60) as well as those on normal human peripheral blood monocytes.