We next examined serum-neutralizing antibody titers in the infected mice; ZH501 was used for the neutralizing antibody assay. For each virus group, 19 to 23 mice were used. As shown in Fig. 5 , all mice that were inoculated i.p with 10 5 PFU of rZH501-M847-A, ZH501 or a mixture of 10 5 PFU of rZH501-M847-G and the same titer of rZH501-M847-A had less than detectable levels of neutralizing antibodies until day 3 p.i. In contrast, rZH501-M847-G-infected mice showed low, but detectable levels of neutralizing antibodies early in infection; moreover, a low titer of neutralizing antibody was detected as early as 1 day p.i. in some of rZH501-M847-G-infected mice. Although neutralizing antibody titers increased in infected mice after day 5 p.i., there was a trend that rZH501-M847-G-infected mice had higher neutralizing antibody titers than did other infected mice. No neutralizing antibodies were detected in sham-infected mice (data not shown).