Pasteurella multocida is an animal parasite of considerable economic and veterinary importance. The organism produces both capsular polysaccharide and somatic (lipopolysaccharide) antigens used to serotype the organisms. Correlations exist between antigenic structure, host susceptibility and strain virulence. The host response seems to be predominantly polymorphonuclear in nature, with little evidence of a mononuclear component in the host response to infection. In the absence of specific opsonins, phagocytosis rates are very slow, the organisms multiplying freely in an essentially extracellular environment. Passive transfer studies indicate that acquired resistance is humorally modiated, presumably by promoting phagocytosis withing the peritoneal cavity. There is, however, no sign of early increased local bactericidal action within the peritoneal cavity; protection seems rather to be due to an inhibition of the rapid spread by the organisms to the bloodstream and other reticuloendothelial organs. Viable attenuated oral vaccines are effective for chicken and turkey poults, both experimentally and under field conditions. The nature of the protective immunoglobulins and their mode of action against an orally induced infection has not been widely examined as yet. Effective protection can be achieved in birds, cattle and experimental animals with a variety of killed vaccines (especially when presented in a suitable adjuvant). The lack of quantitative growth data in suitably vaccinated animals and the absence of definitive studies of the bactericidal mechanisms involved in the expression of this acquired resistance continue to limit our present understanding of defense against this important animal disease.