Methods have been presented for alum-precipitation of the protective antigen of Bacillus anthracis. The product induced effective immunity in rabbits, guinea pigs, and monkeys. Significant immunity persisted for more than three months in rabbits and for more than 14 months in monkeys. Immunity in monkeys was effective against both intracutaneous and respiratory challenge. Alum-precipitated antigen deteriorated slowly when stored at 4°C, but considerable potency remained after storage for approximately two years. The product was devoid of toxicity when tested in mice, guinea pigs, rabbits, and monkeys. Injection of the antigen into man was well tolerated; no serious reactions were encountered and moderate local reactions were infrequent.