We examined the virulence of rZH501-M847-G, rZH501-M847-A and ZH501 by intraperitoneal (i.p.) inoculation of 10 0 , 10 1 , 10 2 , 10 3 , 10 4 or 10 5 plaque-forming units (PFU) of each virus into five 5-week-old female CD-1 mice. Hanks' balanced salt solution (HBSS) was inoculated into control mice. These mice were observed daily for 28 days p.i. (Fig. 3) . None of the HBSSinoculated mice died, while inoculation of 10 2 to 10 5 PFU of ZH501 resulted in the death of all of the mice within 13 days p.i., and one and three mice survived after inoculation of 10 1 and 10 0 PFU, respectively. All mice that were inoculated with 10 2 to 10 5 PFU of rZH501-M847-A died within 9 days p.i., and infection with 10 1 and 10 0 PFU resulted in the survival of one and four mice, respectively. To our surprise, none of the rZH501-M847-G infected mice died after inoculation of 10 0 to 10 2 PFU, four out of five mice survived after inoculation of 10 4 and 10 3 PFU, and one mouse survived after infection with 10 5 PFU. These data demonstrated that ZH501 and rZH501-M847-A were highly virulent to mice, whereas rZH501-M847-G had reduced mouse virulence.