The regulation of B cell death plays roles in the selection of Ag-specific B cells in humoral immune responses, controlling B cell homeostasis and perhaps limiting transformation. The present work addresses whether CD95 induces tonsillar B cells to undergo apoptosis and, if so, whether contact-dependent CD40-L:CD40 signaling can rescue tonsillar B cells from CD95-induced apoptosis. CD95 triggering by anti-CD95 mAb (APO-1) was studied in human tonsillar B cell populations that were separated by density centrifugation into fractions enriched for either low density, CD38+ B cells or high density, resting B cells. Low density tonsillar B cells express CD95 and undergo anti-CD95-mediated apoptosis by analysis of cellular morphology or DNA fragmentation by TUNEL assay. The induction of apoptosis in low density tonsillar B cells by anti-CD95 mAb is inhibited by CD40 signals provided by stably transfected CD40-L+ 293 cells, but not by control transfected 293 cells (expressing CD8). In addition, the rescuing effect of CD40-L+ cells is inhibited specifically by anti-CD40-L (mAb 5c8). The counteracting effects of CD95 and CD40 signaling were also studied in Ramos 2G6, a homogeneous B cell tumor line of germinal center phenotype that expresses CD95 and CD40. Similar to the behavior of low density tonsillar B cells, Ramos 2G6 undergoes anti-CD95-mediated apoptosis, which is prevented by CD40-mediated rescue. These data show that CD95 induces apoptosis in low density tonsillar B cells and that CD40-L:CD40 interactions rescue low density tonsillar B cells or the B cell tumor Ramos 2G6 from CD95-induced apoptosis, and suggest roles for CD95 and CD40 in B cell death and selection, respectively.