Histamine H-1 receptors on peripheral human mononuclear cells were characterized by radioligand binding of the H-1 receptor antagonist [3H]pyrilamine to lymphocyte-rich preparations. Simultaneous computerized analyses of sixteen separate equilibrium-binding assays indicated the presence of two distinct classes of binding sites with dissociation constants (Kds) of 4 +/- 1 nM and 55 +/- 9 microM and binding capacities of 21 +/- 7 fmol and 117 +/- 15 pmol/million cells, respectively. Competition binding curves for displacement of [3H]pyrilamine binding by histamine receptor agonists and antagonists also indicated the presence of multiple binding sites for the H-1 receptor. Further, the ED50 values determined for histamine receptor agonists and antagonists were entirely consistent with the expected rank order of potency for interactions with H-1 receptors. Thus, human mononuclear cells have a large number of H-1 receptors that exhibit two distinct binding sites, and the Kds for these sites are within the range of histamine concentrations achieved either in physiologic states or after mast cell (or basophil) degranulation.