INSULIN receptors have been demonstrated on various human white blood cells including monocytes^1,2 granulocytes³ and cultured lymphoblastoid cells⁴. Of these, the insulin receptor of the monocyte, a circulating macrophage, has been the most useful in the study of human disease^5–9, although insulin has no known effect on macrophage function. We demonstrate here that cultured mouse macrophages¹⁰ and macrophage-containing cell populations from normal mouse spleens both possess specific receptors for insulin with properties indistinguishable from those of the human monocyte, and that insulin, at physiological concentrations, inhibits the ability of these cell populations to lyse antibody-coated red cells (antibody-dependent cell-mediated cytotoxicity or ADCC).