Triggering of the Ig receptor (IgR) induces the activation in multiple intracellular signal transduction reactions including protein tyrosine phosphorylation, activation of phospholipase C, increased inositoltriphosphate, increased diacylglycerol, intracellular Ca2+ mobilization, and activation of protein kinase C. The IgR-complex, composed of mu-chain, L chain, Ig-alpha (MB-1), and Ig-beta (B29) proteins, is a functional unit both for expression of IgR and for signal transduction into cells, possibly by physical association with the down-stream functional molecules. An important functional motif ((D or E)-X7-(D or E)-Y-X3-L-X7-Y-X2-(L or I)) in the cytoplasmic domain of MB-1 molecule was shown to bind with several phosphoprotein components including src-type tyrosine kinases and phosphatidylinositol-3 kinase. To further study the functional components, we analyzed the phosphoprotein molecules coprecipitated with MB-1 protein. We found that a 52-kDa protein is coprecipitated with MB-1 protein and is inducibly phosphorylated by the stimulation with PMA. A rat mAb, prepared by immunizing the 52-kDa protein purified from SDS-PAGE, could detect the similar 52-kDa phosphoprotein (p52) expressed on the cell surface. In comparison with the 52-kDa protein in the immunoprecipitate of MB-1, the p52 migrated to the same position on 2-D gel electrophoresis (nonequilibrium pH gradient gel electrophoresis/SDS-PAGE). An in vitro kinase reaction analysis demonstrated that the p52 is tightly associated with the tyrosine kinase molecule(s), one of which is an 80-kDa protein containing an apparent autophosphorylation activity. These molecules would provide the informations of the down-stream molecules in the cascade reactions of the IgR-mediated signal transduction.