Kit receptor tyrosine kinase and erythropoietin receptor (Epo-R) cooperate in regulating blood cell development. Mice that lack the expression of Kit or Epo-R die in utero of severe anemia. Stimulation of Kit by its ligand, stem cell factor activates several distinct early signaling pathways, including phospholipase C gamma, phosphatidylinositol 3-kinase, Src kinase, Grb2, and Grb7. The role of these pathways in Kit-induced growth, proliferation, or cooperation with Epo-R is not known. We demonstrate that inactivation of any one of these early signaling pathways in Kit significantly impairs growth and proliferation. However, inactivation of the Src pathway demonstrated the most profound defect. Combined stimulation with Epo also resulted in impaired cooperation between Src-defective Kit mutant and Epo-R and, to a lesser extent, with Kit mutants defective in the activation of phosphatidylinositol 3-kinase or Grb2. The impaired cooperation between the Src-defective Kit mutant and Epo-R was associated with reduced transphosphorylation of Epo-R and expression of c-Myc. Remarkably, restoration of only the Src pathway in a Kit receptor defective in the activation of all early signaling pathways demonstrated a 50% correction in proliferation in response to Kit stimulation and completely restored the cooperation with Epo-R. These data demonstrate an essential role for Src pathway in regulating growth, proliferation, and cooperation with Epo-R downstream from Kit.