Weanling random bred mice inoculated intranasally with 2,500 median tissue culture infectious doses of pneumonia virus of mice (PVM) had recoverable virus in the upper respiratory tract for the first 9 days post-infection. Virus was isolated from the lungs only on day 5. Seroconversion was first detected on day 9, and serum antibody titers continued to rise throughout the period of the study (24 days). Lesions were confined to the nasal cavity of infected mice. Susceptible contacts placed in cages with infected mice did not seroconvert to PVM.