Development of anti-Fas Abs to treat diseases with insufficient Fas-mediated apoptosis has been limited by concern about hepatotoxicity. We report here that hepatotoxicity elicited by anti-Fas Ab Jo2 is dependent on FcγRIIB. Thus, following Jo2 treatment, all FcγRIIB−/− mice survived while 80% of wild-type and all FcR-γ−/− mice died from acute liver failure. Microscopic examination suggests that FcγRIIB deficiency protects the hepatic sinusoidal endothelium, a cell type that normally coexpresses Fas and FcγRIIB. In vitro studies showed that FcγRIIB, but not FcγRI and FcγRIII, on neighboring macrophages substantially enhanced Jo2 mediated apoptosis of Fas expressing target cells. However, FcγRI and FcγRIII appeared essential for apoptosis-inducing activity of a non-hepatotoxic anti-Fas mAb HFE7A. These findings imply that by interacting with the Fc region of agonistic Abs, FcγRs can modulate both the desired and undesired consequences of Ab-based therapy. Recognizing this fact should facilitate development of safer and more efficacious agonistic Abs.