Blimp-1 is a transcriptional repressor able to drive the terminal differentiation of B cells into Ig-secreting plasma cells. We have created mice with a B cell-specific deletion of prdm1, the gene encoding Blimp-1. B cell development and the number of B cells responding to antigen appear to be normal in these mice. However, in response to either TD or TI antigen, serum Ig, short-lived plasma cells, post-GC plasma cells, and plasma cells in a memory response are virtually absent, demonstrating that Blimp-1 is required for plasmacytic differentiation and Ig secretion. In the absence of Blimp-1, CD79b⁺B220⁻ pre-plasma memory B cell development is also defective, providing evidence that this subset is an intermediate in plasma cell development. B cells lacking Blimp-1 cannot secrete Ig or induce μS mRNA when stimulated ex vivo. Furthermore, although prdm1^−/− B cells fail to induce XBP-1, XBP-1 cannot rescue plasmacytic differentiation without Blimp-1.