The suppressors of cytokine signaling (SOCS) proteins are a family of SH2 domain-containing intracellular inhibitors of cytokine signal transduction that act by several different mechanisms. Recent evidence suggests that the action of the SOCS proteins may extend beyond the cytokine receptors to signaling initiated by members of the tyrosine kinase receptor family. In this study, the ability of SOCS-5 to negatively regulate signaling cascades downstream of the epidermal growth factor receptor (EGF-R) has been examined by using an EGF-responsive cell line engineered to constitutively express the EGF-R and SOCS-5 or SOCS-5 mutants. SOCS-5 associated with the EGF-R complex in an EGF-independent manner, and the mitogenic response to EGF of all SOCS-5-expressing cell lines was dramatically inhibited when compared with control cell lines. Furthermore, this effect was abrogated after deletion of the SOCS-5 SOCS box. This result suggests that the inhibition of signaling occurs through enhanced proteasomal degradation of the EGF-R through SOCS box recruitment of E3 ubiquitin ligase activity.