Fowl cholera bacterin prepared from liver of chicken infected with strain 5:A induced satisfactory cross-protection in chickens against challenge exposures to either homologous strain (5:A) or heterologous strains of P. multocida belonged to the same capsular serogroup A (8:A and 9:A). A bacterin prepared from bacteria grown on laboratory media induced only homologous protection against challenge with the vaccinal strain (5:A). It appeared that the cross-protecting factors expressed in-vivo grown P. multocida were host specific. This was evident as the chicken tissue bacterin induced only a homologous protection in mice whereas mouse tissue bacterin induced both homologous and heterologous protection in mice.