p105^CasL (CasL) is a recently identified signaling molecule closely related to the p130^Cas (Crk‐associated substrate) docking protein. CasL has a single Src homology (SH) 3 domain in its N‐terminal portion followed by multiple consensus motifs for binding to SH2 domains. Like original p130^Cas, CasL undergoes tyrosine phosphorylation upon integrin‐mediated cell adhesion. In the present report, we provide direct evidence that CasL is also involved in T cell antigen receptor (TcR)‐mediated signal transduction. In binding studies in vitro using glutathione‐S‐transferase fusion proteins, we have identified 105‐ and 120‐kDa phosphotyrosyl proteins (pp105 and pp120, respectively) tightly bound to the SH2 domain of the Crk adapter protein in the H9 human T cell line after stimulation through the CD3/TcR complex. pp120, but not pp105, also bound to the SH3 of another adapter protein, Ash/Grb2. Immunoblotting with specific antibodies revealed that pp120 and pp105 were identical to the c‐cbl proto‐oncogene product (p120^cbl) and CasL, respectively. Association between Crk and tyrosine‐phosphorylated CasL after TcR stimulation was also confirmed in vivo. CasL phosphorylation induced by TcR ligation reached maximal levels within 2 min and rapidly declined thereafter, whereas the integrin‐dependent response occurred slowly and was more prolonged. Finally, we demonstrated that Crk/CasL association occurred in peripheral blood T lymphocytes in response to TcR engagement. Our findings suggest that CasL is involved in T cell activation signals and resides at a point where two distinct receptor‐mediated signaling pathways converge. This provides one mechanism by which integrins may mediate T cell co‐stimulation.