We have studied the expression and function of Fas antigen on murine B lymphocytes. While Fas was present on only a few B cells in the bone marrow, spleen, lymph node or peripheral blood, its expression could be strongly up-regulated by stimulation with soluble CD40 ligand (CD40L). Treatment with anti-IgM and interleukin-4 (IL-4) alone did not induce significant Fas expression but enhanced CD40L-mediated up-regulation of Fas expression. The T cell-derived signal via CD40 is therefore a potent inducer of Fas expression by B lymphocytes. The sensitivity to Fas-mediated apoptosis was found to depend on the duration of B cell activation. B cells activated for 1 day were resistant to Fas-mediated cell death, whereas B cells activated for 3 days were relatively sensitive. Interestingly, different sensitivity to Fas-mediated death signal was observed in 2-day activated B cells. It was found that B cells stimulated with CD40 L alone were more sensitive to Fas-mediated apoptosis than were cells stimulated with CD40L plus anti-IgM or IL-4, and in particular, the combination of the two. The greater sensitivity exhibited by B cells stimulated with CD40L alone seems to be related to limited activation of these cells in the absence of additional stimulation. Co-stimulation of B cells in the presence of CD40L and anti-Fas antibody resulted initially in activation of B lymphocytes, as reflected by the expression of activation markers and cell growth, but this was followed by growth inhibition and cell death. The data demonstrate that the B cell response can be regulated positively and negatively by signaling through CD40 and Fas antigens, respectively.