Abstract P130cas is a dominant tyrosine phosphorylated protein in v-src-and v-crk-transformed cells. Tyrosine phosphorylation also occurs in response to integrin-mediated cell adhesion. P130cas has a unique structure with multiple SH2 and SH3 binding sites, which makes it a candidate docking protein that might be involved in several signal transduction pathways. Little is known about how p130cas itself is regulated. In this report we present evidence that tyrosine phosphorylated p130cas was rapidly dephosphorylated in several lymphatic cell lines after treatment with calyculin A, a serine/ threonine phosphatase inhibitor. A similar result was obtained with okadaic acid, but higher concentrations and longer incubation times were required. Constitutive phosphorylation as well as receptor-cross linking-induced p130cas phosphorylation was inhibited. Furthermore, the p130cas-Crk association was disrupted by treatment of cells with calyculin A. However, the p130cas-Lyn association was not affected. These results suggest that calyculin A specifically affects SH2 domain-mediated protein-protein interactions and that Lyn does not bind to a susceptible SH2 domain. Furthermore, the data presented is consistent with the existence of a calyculin A-sensitive phosphatase or tyrosine kinase that may be a critical regulator of p130cas tyrosine phosphorylation.