We performed cross-neutralization assays to determine how efficiently antiserum from rZH501-M847-G-infected mice and that from rZH501-M847-A-infected mice could neutralize rZH501-M847-G, rZH501-M847-A, and ZH501. Anti-rZH501-M847-G serum demonstrating an 80% neutralizing antibody titer (PRNT 80 titer) of 1:640 to ZH501 and anti-rZH501-M847-A serum demonstrating a PRNT 80 titer of 1:80 to ZH501 were obtained from rZH501-M847-G-infected mice and rZH501-M847-A-infected mice, respectively. We used as controls, diluent, normal mouse serum and convalescent goat anti-ZH501 serum showing a PRNT 80 titer of 1:5,120 to ZH501. In this assay, virus titers were determined after overnight incubation of 100 ml of approximately 5.0 log 10 PFU/ml of rZH501-M847-G, rZH501-M847-A, and ZH501 with the same volume of undiluted antiserum or diluent at 4uC (Table 1) . Goat anti-ZH501 serum efficiently neutralized all viruses, while incubation of rZH501-M847-A and rZH501-M847-G with the normal mouse serum resulted in small reductions of the virus titers. Anti-rZH501-M847-G serum and anti-rZH501-M847-G serum neutralized the three viruses at different efficiencies. The differences between the virus titers (in log 10 PFU/ml) after incubation with each antiserum and those after incubation with normal serum are shown within parentheses in Table 1 . The anti-rZH501-M847-G serum reduced the titer of rZH501-M847-G by 1.63 log 10 PFU/ml, while it reduced the titer of rZH501-M847-A and ZH501 by 0.82 and 0.52 log 10 PFU/ml, respectively. The anti-rZH501-M847-A serum reduced the titers of rZH501-M847-A, rZH501-M847-G and ZH501 by 1.60, 0.7, and 1.0 log 10 PFU/ml, respectively. These data demonstrated that mouse antiserum neutralized the homologous virus to a similar extent, regardless of the PRNT 80 titers, that antiserum neutralized the heterologous virus equally but to a lesser degree than the homologous virus, and that anti-rZH501-M847-A serum was able to neutralize the titer of ZH501 to a slightly greater degree than anti-rZH501-M847-G serum; however, neither serum neutralized ZH501 equal to the neutralization of their homologous viruses.