Viral infections are potentially associated with the etiology and pathogenesis of multiple sclerosis (MS). It has been speculated that the treatment efficacy of interferon beta (IFN beta) in MS may relate to its anti-viral properties. The study was undertaken to evaluate the in vivo anti-viral effects of IFN beta-1a in patients with MS. Human herpesvirus-6 (HHV-6) was studied as an example for being a latent neurotropic virus. IFN beta used at concentrations of approximately 0.5 microg/ml was shown to significantly reduce in vitro HHV-6 replication in a susceptible T-cell line. Sera derived from 23 MS patients treated with IFN beta-1a were examined for serum cell-free DNA of HHV-6 as an indicator for viral replication and the reactivity of IgM antibodies to a recombinant HHV-6 virion protein containing a known immunoreactive region. The results were compared with those of control sera obtained from untreated MS (n=29) and healthy individuals (n=21). The findings indicated that IFN beta treatment significantly reduced HHV-6 replication as evident by decreased cell-free DNA in treated MS specimens. The results correlated with decreased IgM reactivity to the HHV-6 antigen in treated MS patients compared to untreated controls, suggesting reduced exposure to HHV-6. The findings were confirmed in paired sera obtained from seven MS patients before and after the treatment The study provides new evidence indicating that IFN beta has potent in vivo anti-viral effects that may contribute to the treatment efficacy in MS.