To characterize the binding properties of the Fc receptors on human macrophages, the binding of radiolabeled human IgG1 to peritoneal macrophages was assessed. Cells were obtained at the time of diagnostic laparoscopy from women undergoing evaluation of infertility. Macrophages bound on the average more IgG1 monomer than monocytes but the avidity with which both types of cells bound IgG1 monomer was comparable. By contrast, macrophages bound much more IgG1 dimers than monocytes. Scatchard plots of the binding of dimer to monocytes were linear, but plots of binding to macrophages were markedly curvilinear. This curvilinearity was not an artifact of extensive ligand internalization or catabolism by cells, since 80% of binding was reversible and there was very little catabolism of ligand in the medium. Assuming that the observed curvilinearity was due to the presence of two independent subpopulations of receptors, an objective estimate for the number of receptors per cell and of the avidity with which each subpopulation bound IgG1 dimer was obtained using a previously described computer program (Scatfit). The analysis of the binding of dimer to macrophages from six donors suggested the presence of 42,000 +/- 33,000 high avidity receptors per cell which bind IgG1 dimer with a mean Ka of 2.7 X 10(9) M-1 and 218,000 +/- 127,000 low avidity receptors which bind the same ligand with a Ka of 1.1 X 10(7) M-1. ADCC of IgG antibody-coated sheep red blood cells mediated by macrophages was less readily inhibited by soluble IgG1 monomer than ADCC mediated by peripheral blood monocytes. This provides further evidence for the presence of low avidity receptors which bind monomeric IgG1 poorly and also suggests that these sites are functionally active in triggering antibody-dependent immune clearance.