The B cell antigen receptor complex is a hetero-oligomeric structure composed of antigen binding, membrane immunoglobulin, and transducer-transporter substructures. The transducer-transporter substructure is composed of disulfide-linked dimers of immunoglobulin (Ig)-α and Ig-β/γ subunits that are products of the mb-1(α) and B29 (β/γ) genes. Although the receptor complex associates with Src family kinases that are activated after receptor ligation, the site of interaction of these and other cytoplasmic effector molecules with receptor subunits is unknown. The cytoplasmic tails of Ig-α and Ig-β chains were found to associate with distinct sets of effector molecules. The Ig-α chain cytoplasmic domain bound to the Src family kinases Lyn and Fyn, phosphatidylinositol-3 kinase (PI-3 kinase), and an unidentified 38-kilodalton phosphoprotein; the cytoplasmic tail of Ig-β bound PI-3 kinase and unidentified 40- and 42-kilodalton phosphoproteins. Binding activity was found to occur within a 26-amino acid sequence of Ig-α and Ig-β that contains a motif [(Asp or Glu)-(any amino acid)₇-(Asp or Glu)-Tyr-(any amino acid)₃-Leu-(any amino acid)₇-Tyr-(any amino acid)₂-(Leu or Ile)] previously implicated in signal transduction via other receptors including the Fc_ε receptor I and the T cell antigen receptor. These findings indicate that the subunits act independently to activate distinct second messenger pathways.