B-cell receptor (BCR) signals are essential for B-cell differentiation, homeostasis and negative selection, which are regulated by the strength and quality of BCR signals. Recently, we identified a new adaptor protein, Swiprosin-1, in lipid rafts of B-cell lines that undergo apoptosis after BCR stimulation. During murine B-cell development, Swiprosin-1 exhibited highest expression in immature B cells of the bone marrow, but was also expressed in resting and activated splenic B cells and in non-lymphoid tissue, especially in the brain. Ectopic expression of Swiprosin-1 in the immature murine B-cell line WEHI231 enhanced spontaneous and BCR-induced apoptosis. In contrast, short hairpin RNA (shRNA)-mediated downregulation of Swiprosin-1 impaired specifically spontaneous and BCR-elicited apoptosis, but not BCR-induced G1 cell cycle arrest and upregulation of the cell cycle inhibitor p27(Kip1). In accordance, Swiprosin-1 abundance regulated net cell growth of WEHI231 cell populations through reciprocal regulation of Bcl-xL, but not Bim, thereby controlling spontaneous apoptosis. Swiprosin-1-enhanced apoptosis was blocked through nuclear factor kappaB-activating stimuli, namely B-cell-activating factor of the TNF family, anti-CD40 and lipopolysaccharide (LPS). This correlated with enhanced BCR-induced IkappaB-alpha phosphorylation and degradation in cells expressing a Swiprosin-1-specific shRNA. Finally, ectopic Swiprosin-1 expression enhanced BCR-induced cell death in primary, LPS-stimulated splenic B cells. Hence, Swiprosin-1 may regulate lifespan and BCR signaling thresholds in immature B cells.