c-Src, the prototype of the cytoplasmic, membrane-associated,non-receptor tyrosine kinases, is a co-transducer of mitogenic signals emanating from a number of tyrosine kinase polypeptide growth factor receptors. Examples of such receptors include those that bind the platelet-derived growth factor (PDGF), colony stimulating factor-1 (CSF-1), and epidermal growth factor (EGF). Investigations into the mechanisms by which c-Src contributes to receptor signaling suggest that interactions between the two proteins are bidirectional, i.e., that c-Src can bind, phosphorylate, and activate the receptor, and vice versa. The consequences of these interactions appear to be enhanced phosphorylation of specific substrates. Delineating which cellular proteins are substrates of which tyrosine kinase and determining the consequences of tyrosine phosphorylation on the function of specific substrates are the goals of current investigations. Utilizing the murine C3H10T fibroblast model, in which a panel of wild type and mutant c-Src/EGF receptor overexpressors has been studied for temporal and spatial second messenger responses to EGF, distinctions between substrates of c-Src and the EGF receptor and the effects of tyrosine phosphorylation on substrate function are beginning to emerge. In the 10T model, preferred substrates of c-Src are almost exclusively comprised of those molecules that associate with the actin cytoskeleton or with focal adhesions, such as cortactin, p190RhoGAP, and p130CAS, while preferred substrates of the EGF receptor include the receptor itself, SHC, phospholipase C-gamma and p62DOK. While the major mitogenic signaling pathway is thought to proceed directly from the receptor (through SHC/GRB2/SOS/Ras/Raf/MEK/MAPkinase/Elk1), more evidence is accumulating to suggest that proteins involved in regulating the actin cytoskeleton (such as c-Src substrates) also participate in mitogenesis, either as unique transducers of growth signals and/or as monitors of anti-apoptotic conditions (substratum attachment). How c-Src may contribute to the EGF mitogenic response through tyrosine phosphorylation of or association with its specific substrates is discussed. Cellular Src (c-Src), prototype for a family of intracellular membrane-associated tyrosine kinases, is required for mitogenesis initiated by multiple growth factor receptors, including the receptors for epidermal growth factor (EGF), platelet-derived growth factor (PDGF), colony stimulating factor-1 (CSF-1), and the basic fibroblast growth factor (bFGF). C-Src is also overexpressed and/or activated in many of the same human carcinomas that overexpress members of the EGF receptor (EGFR) family, suggesting that the two types of tyrosine kinases can cooperate during the genesis of human tumors. This review focuses on the role of c-Src in EGF-dependent mitogenesis and tumorigenesis, i.e., on the interactions between c-Src and the receptor and on identification of c-Src substrates, their functions, and the effects of tyrosine phosphorylations on their functions. A synopsis of other mitogenic and signaling systems is also included for comparative purposes.