A clonal hamster beta cell line (HIT) was established by simian virus 40 transformation of Syrian hamster pancreatic islet cells. Cytoplasmic insulin was detected in all cells by indirect fluorescent antibody staining, and membrane-bound secretory granules were observed ultrastructurally. Acidified-ethanol extracts of HIT cell cultures contained hamster insulin as determined by radioimmunoassay, radioreceptor assay, and bioassay. One subclone at passage 39 contained 2.6 micrograms of insulin per mg of cell protein. [3H]Leucine-labeled HIT insulin and proinsulin were identical to islet-derived proteins when compared by NaDodSO4/polyacrylamide gel electrophoresis of immunoprecipitates. HIT cell insulin secretion was stimulated by glucose, glucagon, and 3-isobutyl-1-methylxanthine. Insulin secretion at optimal glucose concentration (7.5 mM) was 2.4 milliunits per 10(6) cells per hr. Somatostatin and dexamethasone markedly inhibited HIT insulin secretion. The HIT cell line represents a unique in vitro system for studying beta cell metabolism and insulin biosynthesis.