The catalytic activity of Src family tyrosine kinases is inhibited by intramolecular interactions between the regulatory SH3 and SH2 domains and the catalytic domain. In the inactive state, the critical alphaC-helix in the catalytic domain is positioned such that the formation of the Glu 310-Lys 295 salt bridge is precluded, Tyr 416 in the activation loop is unphosphorylated, and the SH2 and SH3 domains are unavailable for interactions with other proteins. We found that phosphorylation of the activation loop or mutation of the loop preceding the alphaC-helix activates Src and increases the accessibility of the SH3 domain for ligands. Interaction of the alphaC-helix with the activation loop is a central component of this regulatory system. Our data suggest a bidirectional regulation mechanism in which the regulatory domains inhibit Src activity, and Src activity controls the availability of the regulatory domains. By this mechanism, Src family kinases can be activated by proteins phosphorylating or changing the conformation of the catalytic domain. Once active, Src family kinases become less prone to regulation, implying a positive feedback loop on their activity.