A culture of Pasteurella multocida was isolated from a rabbit exhibiting clinical signs of respiratory pasteurellosis. Organisms from this culture were incubated in the presence of the mutagen n-methyl-n'-nitro-n-nitrosoguandine in order to obtain a streptomycin dependent mutant. A live vaccine was prepared from this mutant, and healthy pasteurella-free rabbits were vaccinated either by intranasal or subcutaneous administration. Either method provided complete protection against homologous, wild-type challenge as evidenced by absence of clinical signs or gross pathology and failure to isolate the organism from mucous secretions of tissues from vaccinated individuals.