Inhibition by seven synthetic 4-hydroxycinnamamide derivatives, ST 271, ST 280, ST 458, ST 494, ST 633, ST 638, and ST 642, of tyrosine-specific protein kinases (tyrosine kinase) of oncogene or proto-oncogene products (p130gag-v-fps, p70gag-actin-v-fgr, pp60v-src, pp60c-src) and epidermal growth factor (EGF) receptor kinase were investigated. ST 638 (alpha-cyano-3-ethoxy-4-hydroxy-5-phenylthiomethylcinnamamide) strongly inhibited more of the tyrosine kinases than any of the other compounds. The susceptibilities of these tyrosine kinases to ST 638 increased in the following order: EGF receptor greater than p70gag-actin-v-fgr greater than pp60c-src greater than p130gag-v-fps, pp60v-src, with 50% inhibitory concentration values of 1.1, 4.2, 18, 70, and 87 microM, respectively. The phosphorylation of the tyrosine residues in particulate fractions from RR1022 cells expressing pp60v-src was inhibited by ST 638 in a dose-dependent way, while it had a negligible effect on the phosphorylations of threonine and serine residues. Kinetic analysis showed that ST 638 competitively inhibited the phosphorylation of an exogenous substrate by the EGF receptor kinase with a Ki of 2.1 microM. ST 638 noncompetitively inhibited autophosphorylation by EGF receptor kinase. These results indicate that ST 638 is a potent and specific inhibitor of tyrosine kinases in vitro, and that its inhibitory activity is caused by competing with the substrate protein for the tyrosine kinase binding site.