Dissection of the CD4 signal transduction pathway has revealed striking similarities with the TCR/CD3 pathway. Furthermore, downstream signaling by CD4 is impaired in cells lacking surface TCR, suggesting a role for the TCR/CD3 complex in CD4 signal transduction. We have investigated the molecular basis for the dependence of CD4 signaling on TCR/CD3 expression. Using the phosphotyrosine binding domains of the Shc adaptor and the Fyn kinase, which both participate in CD4 signaling, as baits, we show that CD4 induces tyrosine phosphorylation of a subset of the proteins phosphorylated in response to TCR/CD3 engagement. The phosphoprotein patterns were dramatically altered in cells defective for TCR/CD3 expression, and were recoverable by reconstitution of correctly assembled TCR, suggesting that CD4 uses TCR/CD3-associated tyrosine kinases to signal. Among the tyrosine kinases associated with the resting TCR/CD3 complex, only Fyn is activated following CD4 engagement. The failure of Fyn to become phosphorylated in cells defective for TCR expression underlines the unique role of TCR/CD3 associated Fyn in CD4 signal transduction. While no calcium mobilization was measurable in cells defective for TCR/CD3 expression in response to CD4 engagement, the Ras/MAP kinase pathway could be partially activated. Thus, CD4 activates at least two signaling pathways, and tyrosine kinases associated with the TCR/CD3 complex are key components of one of these pathways.