Cells grown in culture from rat islet cell tumor (parent cells) and clones obtained from them were used in this study. Parent cells secreted primarily insulin and somatostatin with very small quantities of glucagon. The clones, based on hormone content and secretion, were divided into three phenotypic groups: insulin secreting, somatostatin secreting, and nonsecreting clones. Specific receptors for insulin, glucagon, and somatostatin were demonstrated on parent cells and clones. Parent cells bound 4.12 ± 0.46% insulin, 2.11 ± 0.29% glucagon, and 2.49 ± 1.24% somatostatin per 2 × 106 cells. Characteristic hormone binding patterns were observed in insulin secreting versus somatostatin secreting clones. Insulin secreting clones bound less insulin than somatostatin and other noninsulin- secreting clones (P < 0.02). In contrast, somatostatin secreting clones bound more somatostatin than non-somatostatin-secreting clones (P < 0.05). Somatostatin-secreting clones had a significantly greater number of receptors for all three hormones. The difficulties involved in the interpretation of the quantitative aspects of binding in the presence of continued hormone secretion are discussed. Nonetheless, the presence of receptors on the cells for hormones secreted by the same cells strongly suggests autoregulation. The apparent low affinity of some of these receptors and the presence of receptors for all three islet cell hormones on all islet cells supports the likelihood of paracrine controls.