The immunoglobulin alpha (Ig-alpha)-Ig-beta heterodimer is the signaling component of the antigen receptor complex on B cells (BCR) and B cell progenitors (pre-BCR). A mouse mutant that lacks most of the Ig-alpha cytoplasmic tail exhibits only a small impairment in early B cell development but a severe block in the generation of the peripheral B cell pool, revealing a checkpoint in B cell maturation that ensures the expression of a functional BCR on mature B cells. B cells that do develop demonstrate a differential dependence on Ig-alpha signaling in antibody responses such that a signaling-competent Ig-alpha appears to be critical for the response to T-independent, but not T-dependent, antigens.