The physiologic role of eight CD19 tyrosines was examined in CD19-knockout mice expressing transgenic CD19 constructs. CD19 Y482 and Y513 were essential for normal B cell biology, including differentiation of B1 and marginal zone B cells and for T-dependent and -independent antibody responses. In immunized mice with mutations in CD19 Y482 and Y513, early germinal center B cells appeared normal in phenotype and number, but maturation in the germinal center was defective. This was associated with retarded progression through the cell cycle. Thus, Y482 and Y513 are essential for all functions of CD19 in vivo. Mutation of these reduces proliferation in germinal center B cells, providing a potential mechanism for the failure of maturation, which abrogates antibody responses.