Thirteen strains of distemper virus were isolated in ferrets from the spleens of 31 dogs clinically diagnosed as distemper. Serial passage in developing chicken eggs was attempted with all the strains and 3 were successfully adapted to this host. Propagation in chicken embryos of the egg-adapted virus was studied under various experimental conditions with one of the isolated strains and inoculation into the yolk sac of embryonating eggs incubated for only a few days was found to give the greatest yield of the virus. Inoculation of active virus of the egg-adapted strain produced immunity in ferrets and dogs as revealed by protection test and serum neutralization test. The relationship of the amount of inoculated virus and the extent of acquired immunity was studied.