Sac-like particulate antigens were isolated from avirulent cells of two immunogenically distinct strains of Pasteurella multocida by extraction with cold saline and centrifugation at 105,000 × G. They possessed many of the properties ascribed to endotoxins, i.e., they were toxic, high-molecular-weight, nitrogen-containing, phosphorylated lipopolysaccharides which were readily inactivated under mild acid conditions.

Almost 100% of the chickens given either intravenous injections of saline suspensions of the particulate antigens or subcutaneous injections of mineral oil emulsions of them were protected to challenges of live organisms which killed 100% of the controls. The same degree of protection was obtained in mice.

Rabbits injected i.d. with mineral oil emulsions of the particulate antigens gave strong specific antibody responses, but the degree of passive protection of mice by their antisera did not correlate with the amount of antibody. Good protection was obtained with antisera to one of the strains of P. multocida, but not to the other.

Injection of 0.1- to 0.5-mg amounts of the particulate antigens into mice, rabbits or chickens produced moderate to severe toxic effects such as depression and diarrhea; death frequently followed. Chickens showed many of the same signs after injection of fractional milligram amounts of the particulate antigen as have been observed with cases of acute fowl cholera.

Mice were actively and passively protected to the lethal effects of the X-73 antigen, but the degree of active protection was superior to the passive protection obtained with rabbit antiserum.

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