CD95 ligation elicits apoptotic signals in many cell systems. This study analyzes the effect of anti-CD95 mAb on human cells capable of spontaneous and high-rate Ig secretion. Such cells have been induced in vivo and represent a highly mature B cell stage. Addition of the anti-CD95 monoclonal antibody (mAb) CH11 to tonsil B cells inhibited 50-60% of their spontaneous Ig secretion. The effect was exerted early in the culture and could be reversed by a pre-treatment with a neutralizing mAb. N-acetyl-D-sphingosine (C2-ceramide), although not a close analog, also reduced Ig secretion to a similar extent. The inclusion of a tetrapeptide inhibitor for certain interleukin-1beta-converting enzyme proteases prevented the inhibitory effect of CH11 mAb on tonsil B cells. B cells capable of spontaneous Ab secretion obtained from blood of recently-immunized volunteers were also inhibited by CH11 mAb and C2-ceramide. In contrast, bone marrow (BM) B cells capable of spontaneous Ig secretion were unaffected by these agents. This CD95 ligation-mediated inhibition of tonsil and blood Ig-secreting B cells could not be reversed by cytokines with demonstrated activity on these B cells. Human mature B cells induced in vivo are identifiable as CD38hi cells. Flow cytometric analysis revealed that a fraction of tonsil CD38hi cells expressed low levels of CD95. Moreover, about 20% of these cells exhibited basal apoptosis, as defined by annexin V binding. This phenomenon was markedly increased by CD95 ligation. On the other hand, BM CD38hi cells showed neither CD95 expression nor CD95-induced annexin V binding. These data suggest that CD95 ligation might play a role in the control of human humoral responses by inducing apoptosis in susceptible mature B cells.