The binding of 125I-insulin to its receptors was investigated with isolated pancreatic acini obtained from diabetic rats under incubation conditions identical to those used to study the effects of insulin on acinar cell protein synthesis. Binding was specific, time dependent, reversible, and linearly related to the acinar protein content. Degradation of insulin after 30 min of incubation was less than 10% of the total hormone present in the incubation medium. 125I-insulin dissociated from acini with a one-half time of 9 min. Unlabeled insulin at 83.5 nM accelerated the rate of dissociation of labeled insulin. 125I-insulin binding to acini was competitively inhibited by insulin and its analogues in proportion to their biological potencies. Scatchard analysis revealed a major class of insulin-binding sites with a Kd of 1.6 nM; maximal stimulation of protein synthesis was observed when > 90% of these high-affinity receptors were occupied. These studies indicate, therefore, that insulin binding to receptors on pancreatic acini can be correlated with subsequent regulation of biological functions.