The CVS strain of fixed rabies virus causes acute, fatal encephalomyelitis in young adult ICR mice. Variant RV194-2, which was selected from CVS virus in cell culture with a neutralizing antiglycoprotein monoclonal antibody, has a single amino acid change in the glycoprotein. The infections caused by CVS virus and RV194-2 virus were compared in mice for 14 days postinoculation of 5 x 10(7) PFU into the right masseter muscle. All CVS virus-infected mice died (mean time to death, 7.9 days), compared with a mortality rate of 8.5% for RV194-2 virus-infected mice. RV194-2 virus spread to the ipsilateral trigeminal ganglion during the first 2 days postinoculation, and both viruses spread to the ipsilateral motor nucleus of the trigeminal nerve in the pons. Both viruses spread centrifugally and caused infection of bilateral trigeminal ganglia on day 3. The viruses spread throughout the central nervous system (CNS) at similar rates, but CVS virus infected many more neurons than did RV194-2 virus. Rabies virus antigen was observed in only occasional CNS neurons after day 6 of RV194-2 virus infection. By this time, CVS virus had caused severe widespread infection. In this model, virulence depends on improved efficiency of viral spread between CNS neurons rather than the rate of spread or topographical distribution of the infection.