c-Src binds to Raf1, the interaction involves the SH2 and SH3 domains of c-Src and requires serine phosphorylation of Raf1. Coexpression of Raf1 and c-Src in Sf9 cells results in c-Src/Raf-1 complexes, tyrosine phosphorylation of Raf-1, and stimulation of Raf-1 kinase activity. Tyr-340 and Tyr-341 were found to be the major tyrosine phosphorylation sites of Raf1 when coexpressed with activated tyrosine kinases. However, the significance of tyrosine phosphorylation under physiological conditions remains unclear, as tyrosine phosphorylation of endogenous Raf-1 following activation has been disputed and may be limited to cells of hematopoietic origin.