The complement receptor 2 (CR2 or CD21) can be found in non-covalent association with the Blymphocyte specific CD19 complex at the surface of mature human B cells. Upon ligation of the B cell antigen receptor complex (BCR), members of the CR2-CD19 complex may associate with membrane immunoglobulin (mlg). Moreover, CD19 and CD21 ligands, either murine mAb, C3d fragments or Epstein—Barr virus, are known to have profound effects on B cell activation. We here show that CD19 is tightly linked to the non-receptor src kinase Lyn and that the CD19 glycoprotein itself serves as a substrate for a yet undefined serine/threonine kinase present within the complex. In the process of antigen recognition, mlg and the CR2-CD19 complex may bind different sites of a complement-opsonized antigenic particle. We hypothesize that in this process, approximation to the BCR allows CD19-associated Lyn kinase to phosphorylate potential substrates within the antigen—receptor complex, thereby effecting its coupling to the intracellular compartment.