A murine monoclonal antibody, KuFc79, has been developed that reacts with human IgG Fc receptors on monocytes, B lymphocytes, and granulocytes. The specificity of KuFc79 for Fc receptors was demonstrated by the ability of Fab fragments to block Fc-mediated phagocytosis by monocytes. This finding was further substantiated by the decrease of KuFc79 binding to monocytes and granulocytes (58 and 70%, respectively) after modulation of surface IgG receptors with aggregated IgG. In addition, Fab KuFc79-conjugated Sepharose was used to isolate functional IgG Fc-binding molecules from soluble extracts of the human monocyte-like cell line U937. By immunoprecipitation, it was further shown that KuFc79 immunoprecipitated molecules of 42,000 and 70,000 daltons from U937 and a human lymphoblastoid cell line, SB. A lower m.w. species of approximately 33,000 was also precipitable from granulocytes. Extensive microheterogeneity of these molecules was noted by isoelectric focusing, which appears to be due to differential sialation.