CD45 is a member of a family of membrane proteins that possess phosphotyrosine phosphatase activity, and is the source of much of the tyrosine phosphatase activity in lymphocytes. In view of its enzymatic activity and high copy number, it seems likely that CD45 functions in transmembrane signal transduction by lymphocyte receptors that are coupled to activation oftyrosine kinases. The B cell antigen receptor was found to transduce a Ca 2+-mobilizing signal only if cells expressed CD45. Also, both membrane immunoglobulin M (mIgM) and CD45 were lost from the surface of cells treated with antibody to CD45, suggesting a physical interaction between these proteins. Finally, CD45 dephosphorylated a complex of mIg-associated proteins that appears to function in signal transduction by the antigen receptor. These data indicate that CD45 occurs as a component of a complex of proteins associated with the antigen receptor, and that CD45 may regulate signal transduction by modulating the phosphorylation state of the antigen receptor subunits.