Experiments to determine the optimum conditions for propagation of the virus of atypical pneumonia in chick embryos are described. Variations in the activity of infected chick embryo material were investigated. The highest dilution of chick embryo suspension producing pulmonary lesions in hamsters and cotton rats is not over 10(-3). Dilutions of 10(-4) infect chick embryos. The virus is unstable at room temperature and also loses activity when stored in a dry-ice refrigerator unless the suspensions are kept in sealed glass tubes. Filtration experiments indicate a maximum particle size of 180 to 250 mmicro. The virus propagated in chick embryos produces pulmonary lesions in hamsters and cotton rats which have been immunized to their own non-bacterial agents inducing pulmonary lesions. Of these, the pneumonia virus of hamsters most frequently causes intercurrent respiratory infections, and methods of controlling epizootics due to this agent are described.