AbstractInsulin receptor was examined as a substrate for the multipotential protein kinasc casein kinase I. Casein kinase I phosphorylated partially purified insulin receptor from human placenta as shown by immunoprecipitation of the complex with antiserum to the insulin receptor. Analysis of the phosphorylated complex by polyacrylamide gel electrophoresis under nonreducing conditions showed a major phosphorylated band at the position of the α2β2 complex. When the phosphorylated receptor was analyzed on polyacrylamide gels under reducing conditions, two phosphorylated bands, Mr 95,000 and Mr 135,000, were observed which corresponded to the α and β subunits. The majority of the phosphate was associated with the β subunit with minor phosphorylation of the α subunit. Phosphoamino acid analysis revealed that casein kinase I phosphorylated only seryl residues. The autophosphorylated α2β2 receptor purified by affinity chromatography on immobilized O‐phosphotyrosyl binding antibody was also a substrate for casein kinase I. Reduction of the phosphorylated α2β2 receptor indicated that casein kinase I incorporated phosphate into seryl residues only in the β subunit.