Tyr8-SP has been radioactively labeled with 125I to a specific activity of 200 microCI/microgram. This tracer has been found to be rapidly and reversibly bound in a specific way to dispersed pancreatic acinar cells. The specific binding is saturable and dependent on incubation temperature. At 37 degrees C 125I-tyr8-SP is bound to a less degree than at lower temperatures since the peptide is rapidly degraded at 37 degrees C. Native SP inhibits binding of tracer. Results have been analyzed according to Scatchard and suggest 2 functionally distinct types of binding sites; a small number of sites with high affinity and a larger number of sites with low affinity for SP. SP-sites also bind eledoisin and physalaemin which are peptides similar to SP in terms of chemical structure and biological activities. The concentration range in which SP, physalaemin, nd eledoisin inhibit binding of 125I-tyr8-SP correlates well with the range in which these peptides stimulate biochemical processes like outflux of 45Ca, accumulation of cyclic GMP and release of amylase from acinar cells.