Abstract pp 125FAK is a tyrosine kinase localized at the site of focal contacts where it is believed to play an important role in their formation in normal cells. Transformation of cells by pp60 v-src leads to altered cell morphology associated with the disassembly of focal contacts and concomitant increase in tyrosine phosphorylation of pp 125FAK. The precise pathway that regulates the activity of pp 125FAK has not been identified. We investigated the possible interaction between pp 125FAK and p56 lck, an src-like kinase, and found that both p56 lck and PMA treatment tyrosine-phosphorylate and activate pp 125FAK. The in vitro phosphorylation of pp 125FAK by p56 lck is physiological as previously shown by tryptic map analysis [Sharma CP, Goldmann WH. Phosphorylation of (ABP-280; filamin) by tyrosine kinase p56 lck modulates actin filament cross-linking. Cell Biol Int 2004; in press.]. Both, purified p56 lck and PKC phosphorylate pp 125FAK but phosphorylation of pp 125FAK by PKC does not regulate tyrosine phosphorylation, whereas phosphorylation by p56 lck increases the tyrosine phosphorylation by a factor of 4–5. PKC phosphorylation of pp 125FAK does not increase its activity but attenuates the phosphorylation of pp 125FAK by p56 lck, whereas p56 lck phosphorylation only increases the autokinasing activity of pp 125FAK when already activated. These results strongly suggest that not only pp60 v-src but also p56 lck phosphorylate and activate pp 125FAK.