A study of Sendai virus infection in adult mice (2 to 3 months of age) showed that the inbred strains were more susceptible to infection than randomly bred Swiss white mice and that virus could be isolated from inbred strains for as long as 21 days postinfection. For this reason, these mouse strains (C57B1/6J [black] and C57Br [brown]) were selected for the study of intrauterine transmission of virus. The major effect of infection was a decreased weight of both embryos at 16 days of gestation and newborn mice. Virus was isolated from 17 to 20% of the embryos and at least 20 to 30% of the newborns from intravenously infected mothers. Fluorescent-antibody studies showed that the virus was widely distributed in the tissues of both embryos and newborns, including the central nervous system.