Insulin receptors on RINm5F cell membranes (an insulin-producing rat pancreatic cell line) were studied. To study the insulin receptor alpha-subunit, 125I-labelled photoreactive insulin was covalently bound to the membranes in the absence or presence of unlabelled insulin. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis under reducing conditions showed specific labelling of an Mr 130 000 protein. The receptor beta-subunit was studied by using a cell-free phosphorylation assay. Analysis under reducing conditions showed a phosphoprotein of Mr 95 000 whose level of phosphorylation was selectively increased by insulin, and which was specifically immunoprecipitated by antibodies to the insulin receptor. Further, covalent hormone-receptor complexes purified with anti-insulin antibodies were able to undergo autophosphorylation, indicating the existence of operational receptor subunit arrangements. RINm5F cell insulin receptors (and, by analogy, possibly those of native B-cells) thus display structural and functional integrity comparable with those of conventional insulin target cells.