The antigen receptor complex on murine MHC class II-restricted T cells consists of disulfide-linked alpha and beta chains noncovalently associated with four additional polypeptides, two that are endoglycosaminidase F-sensitive, gp26 and gp21, and two that are endoglycosaminidase F-resistant, p25 and p16. We demonstrate here that treatment of murine T cell hybridomas with phorbol 12-myristate 13-acetate results in phosphorylation of p25 and gp21 on serine residues. However, activation of cells by antigen results in the phosphorylation of the gp21 chain and a heretofore unidentified 21 kd protein. This newly defined polypeptide, p21, is specifically immunoprecipitated with the antigen receptor complex, is endoglycosaminidase F-resistant, and is itself part of a disulfide-linked molecule. Unlike antigen-induced phosphorylation of gp21, which occurs on serine residues, phosphorylation of p21 occurs uniquely on tyrosine residues.