More than 75% of a random sample of adult human sera exhibited moderate to high murine pneumonia virus (PVM)-neutralizing activity. There was no correlation between PVM-neutralizing activity and respiratory syncytial virus or parainfluenza type 3 virus-neutralizing activities of the same sera. In children the proportion of sera with moderate to high titres increased with age, indicating early exposure to infection. Seroconversion (i.e. greater than fourfold increase in titre) was observed in four of 108 paired samples of previously undiagnosed respiratory infections. These observations suggest that the human population is frequently exposed to infection with PVM or an antigenically related virus. The sera of patients suffering from Paget's disease of bone tended to exhibit higher than normal PVM-neutralizing titres in comparison with the sera of patients with other bone diseases. Thus, PVM (or an antigenically related virus) resembles some other parainfluenza viruses in being circumstantially associated with Paget's disease of bone.