Insulin stimulates a kinase that phosphorylates tyrosines in the insulin receptor; this kinase is tightly associated with the insulin receptor itself. We now show that the insulin-stimulated casein kinase, present in solubilized, lectin-purified receptor preparations from rat liver, is indistinguishable from the insulin receptor kinase. As with phosphorylation of the insulin receptor, insulin selectively enhanced by 2-3-fold the phosphorylation of tyrosines in casein. The insulin-stimulated activities of both kinases were inactivated at 37 degrees C with the same t0.5 of 5 min and were identically affected by alkylating agents. Both receptor and casein kinase activities were specifically coprecipitated by anti-receptor antibodies or by insulin and anti-insulin antibodies. When the latter type of immune complexes were incubated with an excess of insulin, both kinase activities were quantitatively recovered. We therefore conclude that insulin-stimulated receptor and casein phosphorylations are probably catalyzed by a single enzyme which is tightly associated with the receptor itself. Now, by replacing casein for receptor as substrate, it is possible to measure the enzymatic activity of this receptor-related kinase itself, i.e. independent of the receptor as substrate. Detection of this activity is improved in the presence of certain alkylating agents. Use of artificial substrates (in combination with alkylating agents) is particularly important to dissect the functional components of the receptor complex, to study mechanisms of enzyme regulation and especially in situations where the available receptor for study is limited, e.g. fresh or cultured cells from patients.