During experimental allergic encephalomyelitis (EAE), both blood-borne macrophages as well as activated, resident microglial cells are considered to be involved in inflammatory reactions in the central nervous system (CNS), resulting in the neurological deficits common to EAE. Both cell types can produce multiple mediators of tissue damage, among which are the reactive oxygen species (ROS). In this study we show that macrophages and microglial cells, isolated from the CNS of Lewis rats with clinical signs of EAE, exhibited significantly elevated spontaneous and phorbol myristate acetate (PMA)-inducible levels of ROS compared to similar cells isolated from healthy controls, sham (complete Freund's adjuvant, CFA)-immunized rats as well as rats sacrificed before the manifestation of clinical signs of EAE. However, during clinical EAE, peripheral blood mononuclear cells (PBMC) did not show increased spontaneous nor PMA-inducible ROS production compared to controls. In vivo treatment of EAE with catalase, which scavenges the ROS H2O2, markedly suppressed the severity of the disease as compared to sham (albumin)-treated controls. In contrast, superoxide dismutase had no effect on clinical signs. Our studies point at a putative functional role for ROS, and in particular H2O2, in the pathogenesis of EAE.