Radioiodinated recombinant human interferon-gamma (IFNγ) bound to human monocytes, U937, and HL60 cells in a specific, saturable, and reversible manner. At 4°C, the different cell types bound 3,000-7,000 molecules of IFNγ, and binding was of comparable affinity (K(a) = 4-12 x 108 M-1). No change in the receptor was observed after monocytes differentiated to macrophages or when the cell lines were pharmacologically induced to differentiate. The functional relevance of the receptor was validated by the demonstration that receptor occupancy correlated with induction of Fc receptors on U937. Binding studies using U937 permeabilized with digitonin showed that only 46% of the total receptor pool was expressed at the cell surface. The receptor appears to be a protein, since treatment of U937 with trypsin or pronase reduced 125I-IFNγ binding by 87 and 95%, respectively. At 37°C, ligand was internalized, since 32% of the cell-associated IFNγ became resistant to trypsin stripping. Monocytes degraded 125I-IFNγ into trichloroacetic acid-soluble counts at 37°C but not at 4° C, at an approximate rate of 5,000 molecules/cell per h. The receptor was partially characterized by SDS-polyacrylamide gel electrophoresis analysis of purified U937 membranes that had been incubated with 125I-IFNγ. After cross-linking, the receptor-ligand complex migrated as a broad band that displayed an M(r) of 104,000 ± 18,000 at the top and 84,000 ± 6,000 at the bottom. These results thereby define and partially characterize the IFNγ receptor of human mononuclear phagocytes.