Insulin receptors have been demonstrated on mononuclear leukocytes prepared by centrifugation of buffy coats from normal blood donors on Ficoll-Hypaque gradients. The cell type that specifically binds insulin in this mixture of lymphocytes and monocytes has never been clearly identified, although it was assumed to be the lymphocyte since this cell constitutes about 80% of the population. In the present studies, insulin-binding assays were performed on the mononuclear leukocyte preparation before and after selective depletion or enrichment for monocytes using glass wool or Sephadex G-10 adherence columns. The amount of 125-I-labeled insulin specifically bound correlated significantly with the number of monocytes but not with the number of B or T lymphocytes. Approximately 90% of the specific insulin binding of this preparation could be accounted for by its content of monocytes. The amount of binding was unaffected by phagocytosis of latex particles or by metabolic inhibitors added to prevent endocytosis. Autoradiograms made on smears of whole peripheral blood and mononuclear leukocytes demonstrated that all of the cells that bound 125-I-labeled insulin were large mononulcear cells, 85-90% of which could be identified as monocytes by morphological criteria or by the functional criterion of latex particle ingestion. Since insulin receptor concentration may be altered in disease states in man, it is essential, when using this cell population for detecting such changes, to quantitate the number of monocytes in the preparation so that the insulin-binding data can be appropriately interpreted.