We have studied serine phosphorylation of the β-subunit of highly purified human placental insulin receptors. Each purification step was analyzed with respect to phosphotyrosine and phosphoserine content, incorporated in the β-subunit of the insulin receptor. Independent of the purification state the analysis of the phosphoamino acids of the insulin receptor β-subunit showed tyrosine and serine phosphorylation in an insulin dependent manner. In the presence of insulin up to seven phosphates per αβ-half receptor, indicating a ratio of Tyr(P) and Ser(P) of approximate 3:1 were incorporated, while in the absence of the hormone this ratio did not exeed 1:10. Comparison of the phosphorylation reactions on tyrosine and serine residues makes it highly probable that both phosphoryltransfer reactions obey the same hormone dependence. Half maximal incorporation of total phosphate in the receptor protein was about 5 minutes in contrast to the half maximal serine phosphorylation of about 8 minutes. Our data corroborate that autophosphorylation of serine residues is an intrinsic activity of the receptor kinase itself suggesting a dual-specificity type protein kinase.