To know the kinetics and titers of virus replication in various organs, mice were inoculated i.p. with 10 5 PFU of rZH501-M847-G, rZH501-M847-A, ZH501 and a mixture of 10 5 PFU of rZH501-M847-G and the same titer of rZH501-M847-A. Virus titers in livers, spleens, kidneys, sera and brains were determined from 1 to 7 days p.i.; two to sixteen mice were used for each time point (Fig. 4) . The mice infected with ZH501, rZH501-M847-A, and the mixture of rZH501-M847-G and rZH501-M847A exhibited rapid, efficient virus replications in the liver early in infection, high titers of viremia from 1 to 5 days p.i. and a gradual increase of efficient virus replication in the brain, where the virus titers exceeded 10 6 PFU/g at day 6 p.i. Efficient virus replication also occurred in the spleens and the kidneys, and all mice died by day 8 p.i. The mice infected with rZH501-M847-G also showed rapid virus replication in the liver, and yet the maximum liver virus titer was approximately 10 times lower than that in the mice infected with rZH501-M847-A, ZH501 and the mixture of rZH501-M847-G and rZH501-M847-A. Also rZH501-M847-G titers in the sera, kidneys, spleens and brains were substantially lower than the virus titers in the corresponding organs of mice that were infected with rZH501-M847-A, ZH501 and the mixture of rZH501-M847-G and rZH501-M847-A. Unpaired-t-test results showed that virus titers at days 1 to 3 in the serum and the liver, days 2 to 4 in the spleen, days 2 to 5 in the kidney and days 2 to 6 in the brain of rZH501-M847-G-infected mice were statistically lower (p,0.01) than the titers in the corresponding organs of rZH501-M847-A-infected mice.