A comparative study of the consequences of Parainfluenza type 1 (Sendai) virus infection in inbred (C57B1/6J, C57Br, CBA, DBA) strains and a randomly bred (Swiss white) strain of mice showed significant mortality in the inbred strains but not in the randomly bred ones. This difference may be partly related to the high levels of virus growth obtained in the lungs of the inbred strains contrasted to little or no virus growth in the lungs of the Swiss white mice. A similar difference between these mice was found in the incidence of virus involvement of a variety of mouse tissues. These differences in mortality and in vivo growth of virus were not clearly mimicked in antiviral-antibody production in these different mouse populations.