Engagement of many cell surface receptors results in tyrosine phosphorylation of an overlapping set of protein substrates. Some proteins, such as the adaptor protein Shc, and a frequently observed Shc-associated protein, p145, are common substrates in a variety of receptor signaling pathways and are thus of special interest. Tyrosine-phosphorylated Shc and p145 coprecipitated with anti-Shc antibodies following B cell antigen receptor (BCR) cross-linking or interleukin-4 (IL-4) receptor activation in B cells, and after lipopolysaccharide (LPS) treatment or IgG Fc receptor (Fc gamma R) cross-linking in macrophages. In the case of BCR stimulation, we have shown that this represented the formation of an inducible complex. Furthermore, in response to LPS activation or Fc gamma R cross-linking of macrophages and BCR cross-linking (but not IL-4 treatment) of B cells, we observed a similar tyrosine-phosphorylated p145 protein associated with the tyrosine kinase Syk. We did not detect any Shc associated with Syk, indicating that a trimolecular complex of Shc, Syk, and p145 was not formed in significant amounts. By several criteria, the Syk-associated p145 was very likely the same protein as the previously identified Shc-associated p145. The Syk-associated p145 and the Shc-associated p145 exhibited identical mobility by SDS-polyacrylamide gel electrophoresis and identical patterns of induced tyrosine phosphorylation. The p145 protein that coprecipitated with either Shc or Syk bound to a GST-Shc fusion protein. In addition, a monoclonal antibody developed against Shc-associated p145 also immunoblotted the Syk-associated p145. The observations that p145 associated with both Shc and Syk proteins, in response to stimulation of a variety of receptors, suggest that it plays an important role in coordinating early signaling events.