Protein tyrosine kinases are crucially involved in the control of cell proliferation. Therefore, the regulation of their activity in both normal and neoplastic cells has been under intense scrutiny. The product of the MET oncogene is a transmembrane receptorlike tyrosine kinase with a unique disulfide-linked heterodimeric structure. Here we show that the tyrosine kinase activity of the MET-encoded protein is powerfully activated by tyrosine autophosphorylation. The enhancement of activity was quantitated with a phosphorylation assay of exogenous substrates. It involved an increase in the V max of the enzyme-catalyzed phosphotransfer reaction. No change was observed in the K m (substrate). A causal relationship between tyrosine autophosphorylation and activation of the kinase activity was proved by (i) the kinetic agreement between autophosphorylation and kinase activation,(ii) the overlapping dose-response relationship for ATP,(iii) the specificity for ATP of the activation process,(iv) the phosphorylation of tyrosine residues only, in the Met protein, in the activation step,(v) the linear dependence of the activation from the input of enzyme assayed, and (vi) the reversal of the active state by phosphatase treatment. Autophosphorylation occurred predominantly on a single tryptic peptide, most likely via an intermolecular reaction. The structural features responsible for this positive modulation of kinase activity were all contained in the 45-kDa intracellular moiety of the Met protein.