Complement-fixing antibodies to parainfluenza 3 virus were found in Hartley strain [Cds: (HA)] guinea pigs from the authors' production colony. The prevalence and distribution of these antibodies were determined by testing guinea pigs of five age categories: 4 weeks, 8 weeks, 12 weeks, 6 months to 1 year, and over 1 year of age. Forty-seven percent (28 of 60) were positive to parainfluenza 3 antigen. Positive reactors were found in all age groups except those 8 weeks old. The 12-week-old group had the highest titers; the group over 1 year of age had the highest percentage of positives (92%). When 8-week-old guinea pigs were isolated, 55% were positive at some time between 8 and 34 weeks of age. The titers characteristically rose rapidly and then dropped slowly to low or undetectable levels. Four pairs of breeders over 6 months of age (most of whom were positive for parainfluenza 3 virus antibodies and, therefore, presumed to be immune to the virus) were isolated and allowed to breed. Their offspring were found to be free of complement-fixing antibodies to parainfluenza 3 virus.