The results of two phase III clinical trials have recently shown that interferon-beta (IFNbeta) is effective in the treatment of relapsing-remitting multiple sclerosis (RRMS). Treatment with IFNbeta results in a significant decrease in the rate of clinical relapse and a marked delay in progression to disability compared to placebo-treated control patients. In the present study, we demonstrate similar therapeutic effects after treating (SWR X SJL)F1 mice with IFNbeta at the onset of clinical signs of experimental autoimmune encephalomyelitis (EAE), a disease animal model widely used in MS studies. EAE was actively induced by immunization of (SWR X SJL)F1 mice with the immunodominant encephalitogenic peptide 139-151 of myelin proteolipid protein (PLP). In blinded testing, mice treated with IFNbeta at EAE onset showed a delay in progression to clinical disability as determined by marked improvement with time in mean clinical score, significant delay in onset of relapse, and significant decrease in exacerbation frequency compared to placebo-treated control mice. The therapeutic effect of IFNbeta was accompanied by a significant inhibition of delayed-type hypersensitivity (DTH) but not proliferation in response to the priming PLP 139-151. In addition, IFNbeta treatment resulted in an overall decrease in severity of both inflammation and demyelination in the central nervous system. These results mimic in an autoimmune animal model the effectiveness of IFNbeta treatment observed in MS. Moreover, our study suggests that anti-viral properties of IFNbeta are not essential for producing therapeutic effects in autoimmune demyelinating disease, and that the efficacy of IFNbeta in the treatment of MS may be due to inhibition of autoreactivity.