The major histocompatability class II heterodimer (class II) is expressed on the surface of both resting and activated B cells. Although it is clear that class II expression is required for Ag presentation to CD4 + T cells, substantial evidence suggests that class II serves as a signal transducing receptor that regulates B cell function. In ex vivo B cells primed by Ag receptor (BCR) cross-linking and incubation with IL-4, or B cell lines such as K46-17μmλ, class II ligation leads to the activation of protein tyrosine kinases, including Lyn and Syk and subsequent phospholipase Cγ-dependent mobilization of Ca 2+ . In this study, experiments demonstrated reciprocal desensitization of class II and BCR signaling upon cross-linking of either receptor, suggesting that the two receptors transduce signals via common processes and/or effector proteins. Because class II and BCR signal transduction pathways exhibit functional similarities, additional studies were conducted to evaluate whether class II signaling is regulated by BCR coreceptors. Upon cross-linking of class II, the BCR coreceptors CD19 and CD22 were inducibly phosphorylated on tyrosine residues. Phosphorylation of CD22 was associated with increased recruitment and binding of the protein tyrosine phosphatase SHP-1. Similarly, tyrosine phosphorylation of CD19 resulted in recruitment and binding of Vav and phosphatidylinositol 3-kinase. Finally, co-cross-linking studies demonstrated that signaling via class II was either attenuated (CD22/SHP-1) or enhanced (CD19/Vav and phosphatidylinositol 3-kinase), depending on the coreceptor that was brought into close proximity. Collectively, these results suggest that CD19 and CD22 modulate class II signaling in a manner similar to that for the BCR.