Chick embryos infected intra-allantoically with nonvirulent staphylococci are protected from death due to infection with virulent staphylococci. The protection is associated with a delay in growth of the challenge strain and a delay in the production of toxic substances in the allantoic fluid. The protection is influenced by the number of bacteria in the protecting and challenge inocula and by the interval between the administration of the protecting and challenge strains. Protection cannot be transferred by administration of sterile filtrates of allantoic fluid in which the protecting strain has grown.
