Neonatal ferrets were found to be highly via the intranasal route of inoculation to infection with wild type isolates of parainfluenza viruses. Wild type infection consistently resulted in deaths of these animals in 48 to 72 hours. Autopsy and histopathological findings in lung tissue were consistent with those of viral infection. Virus could consistently be isolated from lung tissues. Using wild green monkey kidney grown viruses and selected passage levels of egg adapted viruses, virus attenuation markers were observed by comparing infectivity in neonatal ferrets, propagation in cell cultures of human, primate, rabbit and chick embryo tissues, and in chick embryos and interferon induction. Adult pregant dams could be immunized (silently infected) and circulating hemagglutinin-inhibiting antibody could be detected within two weeks of intranasal instillation of wild and low egg passage types of virus. The animal model described could be valuable in studying immunity to parainfluenza virus infections.