Fas ligand (FasL) has been shown to mediate both apoptotic and inflammatory reactions. To rigorously assess the physiological role of different forms of the FasL molecule with regard to these two distinct processes, we isolated stably transfected lymphoma cell lines that expressed either murine wild-type FasL, membrane-only FasL, or functionally distinct forms of soluble FasL. First, the ability of these lines to induce an inflammatory response was assessed in vivo by injecting the transfectants intraperitoneally and measuring subsequent neutrophil extravasation into the peritoneal cavity. Second, lines were assessed by injecting the transfectants subcutaneously and monitoring their growth as solid tumors. Our study clearly demonstrated that the extent of inflammation induced by the transfectants directly correlated with their relative cytotoxic activities. A neutrophil response could only be elicited in mice with intact Fas death domains although Fas expression by the neutrophils was not essential. Lymphoma cells expressing the soluble FasL form corresponding to the natural cleavage product could not trigger apoptosis and did not induce a neutrophil response. In contrast to the other FasL transfectants, these cells survived as tumor transplants. However, expression of soluble FasL was not benign, but actually suppressed the inflammatory response and protected other transfectants from the effector mechanisms elicted by membrane-bound FasL.