Binding of 125I-labeled human GH (hGH) to a cloned rat insulin-producing cell line RIN-5AH in monolayer culture was studied along with some physiological effects of the hormone on these cells. Binding was time and temperature dependent, and steady state binding was observed in 60 min at 37 C with [125I]hGH at 4.2 pM, whereas at 24 C, binding had not reached a steady state after 120 min. The binding was largely reversible, since 80% of initially bound [125I]hGH dissociated from the cells upon incubation in hGH-free buffer for 120 min. Half-maximal binding was obtained when cells were incubated in the presence of 3.0 X 10(-10) M unlabeled hGH. Rat GH as well as human placental lactogen were able to compete for binding sites, but with less affinity. Other non-GH peptides at 6.7 micrograms/ml did not affect [125I]hGH binding. Scatchard analysis revealed curvilinear plots, and approximately 2700 high affinity binding sites were calculated. Culture of RIN-5AH in the presence of 1 microgram/ml hGH for 4 days resulted in an 80% increase in insulin content as well as an 18% increase in cell number and DNA and protein content compared to those in cells cultured in the absence of hGH. The dose dependence of the insulinotropic effect showed that half-maximal and maximal stimulation were observed in cells cultured in the presence of 10 and 100 ng/ml, respectively. Insulin release to the medium during the 4-day culture period was not affected by hGH. These data suggest that GH, through binding to specific receptors in the cell membrane, directly stimulates proliferation and function of pancreatic beta-cells.