In a mouse colony in which lymphocytic choriomeningitis is endemic infection takes place either in utero or shortly after birth. Virus is discharged from infected mice with the nasal secretions and urine. In some mice the infection lasts for several months, and such carriers can transmit the disease to healthy mice by contact. The portal of entry appears to be the nasal mucosa rather than the gastrointestinal tract. Mice infected by contact show no definite symptoms while those infected in utero often do. The disease has persisted in the colony for at least 15 months without change in its character. Mouse stocks differ in their susceptibility to contact infection and the findings given in the paper could be reproduced only with a very susceptible stock. Wild mice (Mus musculus) can be infected by contact, although less easily than our white mice.

The source of the infection in the colony has not been determined. The fact that the serum of the caretaker neutralizes the virus indicates that he has been infected. It seems likely that the virus went from him to the mice rather than vice versa. Other possible sources of infection are considered.