In a large-scale mouse breeder colony persistently infected with Sendai and mouse hepatitis viruses, most adult breeders 8 wk or more of age were shown to have antibodies to both viruses when monitored over a periof of 20 mo. Antibody to Sendai virus, apparently transmitted from the dam, was detected in 76% and 2% of mice aged 3 and 4 wk. respectively, and 64% and 100% of mice aged 6 and 8 wk, respectively. By seroconversion of sentinel cage-mates, a Sendai virus-carrier state was demonstrated with 6-wk-old mice but not with those either 4 wk or 10 wk of age, suggesting that breeder candidates about 6 wk of age may play an important role in establishing and perpetuating Sendai infection in this breeding colony. With mouse hepatitis virus, however, mice aged 4 wk or older seem to be effective transmitters of the virus, while some of these mice were found to have antibody to the virus.