The expression and action of Fas/Fas ligand (FasL) in multiple sclerosis has been postulated as a major pathway leading to inflammatory demyelination. To formally test this hypothesis, C57BL/6-lpr and -gld mice, which due to gene mutation express Fas and FasL in an inactive form, were immunized with myelin oligodendrocyte glycoprotein peptide(35-55). Whereas in wild-type C57BL/6 mice, experimental autoimmune encephalomyelitis (EAE), was chronic/relapsing, EAE in lpr and gld mice was characterized by a lower incidence of disease and a monophasic course. This contrasts with C57BL/6 perforin knockout mice, which showed the most severe form of EAE of all mouse strains tested, the course being chronic relapsing. The difference noted cannot be attributed to an involvement of FasL in oligodendrocyte damage since oligodendrocytes are insensitive to FasL-mediated cytotoxicity in vitro, and since in the acute phase of EAE gld mice also show CD4+ T cell infiltrates with associated demyelination in brain and spinal cord. Unlike oligodendrocytes, astrocytes were killed by FasL in vitro. It remains to be established whether this latter finding explains the different disease course of lpr and gld mice compared to wild-type and perforin knockout mice.