The B cell-specific activator protein (BSAP) is a DNA-binding transcription factor expressed in pro-B, pre-B, and mature B cells, but not in plasma cells. In this study, we explored the role of BSAP in B cell function by assessing how the content of this protein varies in cells driven by proliferative stimuli and, conversely, how artificial manipulation of BSAP activity affects cell proliferation. We found that BSAP activity of nuclear extracts increased when B cells were activated by mitogen (lipopolysaccharide [LPS]), antigen receptor-mediated signaling (surface immunoglobulin D [IgD] cross-linking) or T cell-dependent stimulation (CD40 cross-linking). We could suppress BSAP activity by exposure of B cells to phosphorothioate oligonucleotides antisense to the BSAP translation initiation start site, whereas control oligonucleotides were virtually inactive. Antisense-induced BSAP suppression was associated with a striking reduction in LPS-induced proliferation of splenic B cells and in the spontaneous proliferation of B lymphoma cells (CH12.LX), but the antisense oligonucleotide had virtually no effect on proliferation of two cell lines lacking BSAP: the T lymphoma line EL-4 and the plasma cell line MOPC-315. Overexpression of BSAP in splenic B cells or de novo expression in MOPC-315 plasma cells induced by transfection of a BSAP expression plasmid stimulated cell proliferation. Taken together, these results suggest that BSAP activity is a rate-limiting regulator of B cell proliferation. We also found that treatment with the antisense BSAP oligonucleotide downregulated Ig class switching induced by interleukin 4 plus LPS. This effect may be secondary to reduced proliferation or could be mediated through BSAP binding sites in the IgH locus.