The Triton X-100 solubilized extract of human peripheral blood mononuclear cells, in direct binding studies with 10-9-10-6M [3H]histamine contained both high and moderate affinity sites whose dissociation constants (Kd4.4 × 10-9 aand 6.7 × 10-7 M) were commensurate with basal plasma histamine levels and plasma levels obtained following physiological or mild pathological stimuli, respectively. Binding was enhanced by mM concns of calcium cations and by the protease inhibitor Pepstatin A. It was inhibited by bacitracin, agents interfering with thiol groups, Triton X-100 concns greater than 0.2% and EDTA. Binding was optimal between the pH range of 7.0 and 8.5 and was enriched for in a plasma membrane preparation. Thus the histamine binding sites identified maintained their specific ligand binding properties after solubilization from the cell surface and displayed properties fulfilling the criteria for receptors.