Sendai virus is one of the more prevalent and serious virus infections of rodents. Infection was found in 66% of the mouse, 63% of the rat, 83% of the hamster, and 44% of the guinea pig colonies examined. Twenty-four inbred and outbred strains of mice were tested for their sensitivity to lethal Sendai virus infection. The 129/J mice tested were approximately 25,000-fold more sensitive than SJL/J mice; however, both mouse strains were similarly permissive in support of viral replication in their lung tissues. Histopathological studies revealed that whereas lesions in both sensitive and resistant mice were qualitatively similar, the lesions in the more sensitive 129/J mice appeared earlier, were much more extensive, and persisted longer than in the resistant SJL/J mice. These results suggest that the observed variance in sensitivity is not the result of a genetic restriction on virus infection and replication but rather is the result of a physiological factor(s) possibly related to some aberration of strain difference in the humoral or cell-mediated immune response.