The characteristics of the insulin- and epidermal growth factor (EGF)-stimulated tyrosine-specific protein kinases in a wheat germ lectin-Sepharose-purified preparation of solubilized placenta membranes were compared. The specific activity of the insulin-stimulated kinase in this preparation was 72 nmol/min/mg whereas the specific activity of the EGF-stimulated kinase was 312 nmol/min/mg using a synthetic peptide as the phosphorylatable substrate. The two enzymes showed similar divalent metal ion requirements and nucleotide specificities. In addition, both kinases were inhibited by treatment with N-ethylmaleimide. However, the EGF-stimulated enzyme was more sensitive to modification by this reagent than was the insulin-stimulated kinase. When examined for their ability to utilize a number of different proteins as substrates, the insulin- and EGF-stimulated kinases exhibited similar but not identical substrate specificities. These similarities at the molecular level may be the basis of the similarity between the actions of insulin and EGF at the cellular level.