B cell receptor (BCR) signaling is mediated through immunoglobulin (Ig)α and Igβ a membrane-bound heterodimer. Igα and Igβ are redundant in their ability to support early B cell development, but their roles in mature B cells have not been defined. To examine the function of Igα–Igβ in mature B cells in vivo we exchanged the cytoplasmic domain of Igα for the cytoplasmic domain of Igβ by gene targeting (Igβc→αc mice). Igβc→αc B cells had lower levels of surface IgM and higher levels of BCR internalization than wild-type B cells. The mutant B cells were able to complete all stages of development and were long lived, but failed to differentiate into B1a cells. In addition, Igβc→αc B cells showed decreased proliferative and Ca2+ responses to BCR stimulation in vitro, and were anergic to T-independent and -dependent antigens in vivo.