Mononuclear leukocytes (MNL) were isolated from human blood by Ficoll-Hypaque. These cells were further separated into lymphocyte (L) and monocyte (M) enriched fractions. L contained 99% lymphocytes and M contained 74% monocytes, a threefold enrichment over MNL. Specific binding to somatostatin, glucagon, and insulin was measured in the three fractions. Binding of all three hormones in the M fraction was increased by a factor of 3 compared with MNL and was linear with cell number. Binding of glucagon and insulin to the L fraction was very low while, in contrast, somatostatin binding was substantial and linear with lymphocyte number. Autoradiography confirmed the binding of glucagon to monocytes and of somatostatin to both monocytes and lymphocytes. Somatostatin is the first of the peptide hormones shown to bind to both types of circulating mononuclear cells, perhaps complicating quantification of somatostatin binding in disease states in which differential alteration of binding of lymphocytes or monocytes might occur.