We had previously showed that following intracerebral inoculation of newborn mice, the 910 N and M strains of bovine parainfluenza 3 virus induce a non-lethal hydrocephalus and a lethal disease with marked thymic and splenic atrophy, respectively. Moreover, only the M virus was lethal for 2-week-old mice. In the present study, we demonstrate that the M virus multiplies and spreads in the mouse brain invading the thalamus, hypothalamus and brain stem beyond the ependyma whereas the 910 N virus causes only slight ependymitis. This growth and spread of M virus was blocked by passive immunization 3 days after infection. Mouse embryo brain cell cultures were infected with M and 910 N viruses, about 50 per cent became antigen-positive for M whereas only a small proportion of cells were positive for the 910 N virus. However, the latter did produce higher yeilds of infectious virus than M.