An epizootic of Sendai virus infection occurred in a mouse breeder colony with respiratory signs, mortality, retarded growth in young and prolonged gestation period in adults. Thereafter, the infection persisted in this colony without any clinical signs and with 90 to 100% antibody positivity in adults. Weanlings had maternal antibodies and no pneumonic lesions. Antibodies were hardly detected in 6-week-old mice with high incidence of red hepatization or congestion in their lungs, but mice over 8 weeks of age had antibody without the lesions. After isolating weanlings and pregnant mice with antibodies from the infected colony, the isolated weanlings and offspring from the isolated dams became negative for Sendai virus antibodies.