The targets of receptor protein-tyrosine kinases are characterized by Src homology 2 (SH2) domains, that mediate specific interactions with receptor autophosphorylation sites. SH2-mediated interactions are important for the activation of biochemical signalling pathways in cells stimulated with growth factors. A distinct protein module, the SH3 domain, is frequently found in polypeptides that contain SH2 domains, and is also implicated in controlling protein-protein interactions in signal transduction. Evidence suggesting that SH2 and SH3 domains act synergistically in stimulation of the Ras pathway is discussed.