Association of insulin with the plasma membrane of the pancreatic B-cell was revealed using the high-resolution protein A-gold immunocytochemical approach. The labeling was found to be heterogeneously distributed, the plasma membrane of the actin-rich microvilli being preferentially labeled. Morphometrical evaluation of labeling intensities confirmed the microvilli location of the insulin binding sites. This membrane domain, which also displays the glucose transporter, therefore appears to play important functional roles in the secretory activity of the B-cell. That the autocrine influence insulin exerts on its own secretion is receptor mediated through these insulin binding sites remains, however, to be determined.