By means of fluorescein-labelled antibody, the primary atypical pneumonia virus was found to multiply exclusively in the cytoplasm of the epithelial cells lining the bronchioles and air sacs of developing chick embryos. When 13-day old embryos were inoculated intra-amniotically and incubated at 35 degrees C. for 5 days or longer, over 90 per cent of the inoculated embryos became infected. Between 1954 and 1956, seven strains of PAP virus were isolated from sputums or nasopharyngeal washings in patients during the acute stage of the PAP infection. One strain of virus was isolated from the frozen lung of a patient who died at Boston in 1943. All eight recently isolated strains and the Mac strain isolated by Eaton et al. in California in 1944 were antigenically closely related if not identical. PAP virus is not related antigenically to agents of psittacosis, Q fever, adenovirus (Types 1 to 6), influenza A or B, or PVM.