Weanling mice were infected with avianized H virus of low pathogenicity, and various virological and immunological parameters were examined. In comparative study with other strains, mice infected with F and Str. viruses showed very high virus multiplication in the brain and all the infected mice died. In mice infected with H virus, the virus multiplied and, then, disappeared rapidly in the brain, and the majority of mice survived. CF antigen could be demonstrated in the brain of mice infected with H virus, even after the disappearance of the virus. In mice infected with Fl. H virus, the virus multiplied at such a low level that it was difficult to detect a distinct CF antigen from these mice. Mice infected with H and Fl. H viruses produced high virus-neutralizing antibody and CF antibody in serum. When a higher dose of H virus in the inoculum was employed, the virus multiplied earlier and disappeared earlier in the mouse brain compared with those infected with lower doses. In addition, in these mice, neutralizing antibody were produced earlier in serum. Among mice inoculated with a low dose of H virus, there was a decrease in the incidence of infection, and the onset of illness was delayed. However, their death rate was higher than that of those infected with high doses. No virus was demonstrated in the brain of the dead mice. In the early stage of H virus infection, no inhibitors or incomplete virus particles were found in the brain. When various types of antigen harvested from the brains of mice infected with H virus were examined, the protective ability or ability to produce neutralizing antibody in serum was correlated with the virus titers, and the ability to produce CF antibody in serum was correlated with the CF antigen titer in the brain. The disappearance of the virus from mouse brains infected with H virus in early stage, recovery from clinical illness and the rapid development of virus neutralizing antibody were discussed.