The tyrosine kinases of the Src family were first discovered due to their oncogenic properties. In untransformed fibroblasts, these kinases are activated as cells exists quiescence in response to some growth factors. Using microinjection to introduce catalytically inactive dominant-negative form of cSrc, as well as an antibody that neutralizes cSrc, Fyn and cYes, we have shown that Src kinases are required for DNA synthesis induced by most growth factors (PDGF, EGF, CSF-1, insulin, IGF-1). A functional link between Src kinases and the expression of the transcription factor c-Myc was also shown. In addition to cell growth promotion, some factors induce epithelial cell scattering and this also requires cSrc and cYes activities. However, in contrast to mitogenesis, they do not need novel gene expression for signalling but rather may act by phosphorylating components that regulate the cytoskeleton. Finally, increased Src kinase activities were found in several human carcinomas and we propose that these enzymes are involved in cell invasion.