W e have investigated the effects of the monoclonal antibodies against the cell surface molecule Mac-1 on C3bi-mediated rosetting and IgG-mediated rosetting and phagocytosis by human peripheral blood monocytes. Highly purified M1/70 F(ab')2, used in the fluid phase, inhibited both monocyte functions. Half-maximal C3bi rosette inhibition occurred at a concentration of 2 nM F(ab')2 M1/70. An equivalent decrease in IgG-mediated rosetting required 10 nM M1/70 F(ab')2, and 50% inhibition of IgG-mediated phagocytosis required 7 nM antibody. Mo-1 F(ab')2 inhibited EC3bi binding with an ID50 of 0.3 nM, whereas 50% decrease in IgG-mediated rosetting required 70 nM of this antibody. OKM1 did not inhibit rosettes of sheep erythrocytes opsonized with IgG antibody (EA) at all. F(ab')2 M1/70 did not affect the binding of monomeric human IgG to monocytes, but did substantially decrease the binding of IgG aggregates. Half-maximal inhibition of aggregated IgG binding at 0 degrees C occurred at 8 nM F(ab')2 M1/70, very close to the concentration that caused equivalent inhibition of IgG-mediated phagocytosis. Aggregated IgG inhibited the binding of radiolabeled M1/70 to monocytes by approximately 40%, suggesting that some, but not all Mac-1 molecules were associated with IgG receptors under these conditions. When cells were allowed to adhere to surfaces coated with M1/70 or Mo-1 F(ab')2, C3bi-mediated rosetting was inhibited, but IgG mediated-phagocytosis was unaffected. Moreover, the dose response of inhibition of phagocytosis by fluid-phase F(ab')2, of anti-Mac-1 monoclonals was similar on monocytes adherent to albumin-coated and antibody-coated surfaces. Kinetic experiments showed that even prolonged incubation of monocytes on M1/70 coated surfaces did not lead to inhibition of EA binding nor did these incubations alter the dose response for inhibition of EA binding by fluid-phase M1/70 F(ab')2. This suggested that not all molecules recognized by M1/70 are freely mobile in the plasma membrane. Indeed, only approximately 60% of 125I-M1/70-biding sites were lost even after 4 h when monocytes were adherent to M1/70-coated surfaces. We conclude that some anti-Mac-1 antibodies can inhibit EA binding because of their epitope specificity, independent of any direct interaction with monocyte Fc receptors. This interference with IgG-Fc receptor-mediated binding and ingestion apparently occurs because of antibody binding to a subpopulation of Mac-1 molecules which are associated with IgG Fc receptors and remain on the apical membrane of monocytes adherent to anti-Mac-1-coated surfaces. We suggest that there may be two functionally distinct molecules on human monocytes recognized by M1/70 and Mo-1 that can be distinguished by their mobility in the plane of the monocyte membrane. The more mobile form of Mac-1 is involved in C3bi rosettes, and does not affect IgG-mediated phagocytosis. The other antigen recognized by M1/70 does not diffuse within the plane of the membrane; ligation of the latter molecule by antibody is associated with inhibition of IgG-mediated phagocytosis.