Focal adhesion kinase (FAK) participates in signaling events induced by diverse stimuli including integrin engagement, oncogenic transformation and mitogenic neuropeptides. FAK's signaling function is regulated by tyrosine phosphorylation. The major autophosphorylation site is tyrosine-397, which interacts with the Src homology 2 (SH2) domain of Src-family kinases including Src and Fyn. Full activation of FAK appears to require additional phosphorylation by the associated Src-family kinases. Previously identified Src sites include catalytic domain tyrosines-576 and -577, important for maximal FAK kinase activity, and tyrosine-925, which permits an SH2-mediated association with Grb2. A full understanding of FAK-mediated signaling events will require the identification of all sites of tyrosine phosphorylation. Here we report that tyrosine-861 is the major Src site in the carboxyl-terminal domain of FAK. Phosphotyrosine-861 may function in additional interactions between FAK and SH2-containing proteins.