We noted that neutralizing antibodies were rapidly elicited in rZH501-M847-G-infected mice within 3 days p.i. (Fig. 5) , during which time most of the viruses retained the genotype of rZH501-M847-G (Fig. 8) . Because the importance of neutralizing antibodies for the protection of animals from RVFV infection is well documented [34, 35, 36, 37, 38, 39, 40, 41, 42] , a rapid and efficient generation of neutralizing antibodies probably contributed to low titers of viremia and prevention of systemic infection in rZH501-M847-G-infected mice. It is interesting to note that the antiserum neutralized the heterologous virus equally but to a lesser degree than the homologous virus in the cross neutralization assay (Table 1) . These results were consistent with a report that the region including Glu277 or Gly277 of Gn protein is a major neutralizing epitope site [43] . It is likely that in the rZH501-M847-G-infected mice, rZH501-M847-A-type virus emerged partly because this virus was less susceptible to rapidly generated neutralizing antibodies directed against rZH501-M847-G. We further suspect that in the rZH501-M847-G-infected mice, newly emerged rZH501-M847-A-type virus failed to continuously replicate efficiently after 5 days p.i. in various organs primarily due to an increase in the titers of neutralizing antibodies, resulting in the survival of the mice. The virus titers in rZH501-M847-Ainoculated mice were higher than those inoculated with rZH501-M847-G in many organs during the first 4-5 days p.i., while the neutralizing antibody titers tend to be higher in rZH501-M847-Ginoculated mice than in rZH501-M847-A-inoculated mice (Fig. 5) . These data suggest that rZH501-M847-A replication might have induced immunosuppression. This possibility was consistent with the data that lymphocytes were depleted in the white pulp of mice inoculated with rZH501-M847-A, but not those inoculated with rZH501-M847-G, a finding probably due to apoptosis (Fig. 6 ).