In several independently conducted potency tests in mice, a sample of the new type of formalin-killed Haffkine vaccine, manufactured in Bombay from caseinate broth cultures of virulent Yersinia pestis (strain 195/P), appeared to be immunogenically identical to the old type of Haffkine vaccine. Antigenic differences, however, became evident during trials in man. A significant booster effect on the mouse protection index of serum antibody was imparted by the new vaccine, which contained fourfold more Fraction I than the old vaccine. The serologic responses, including markedly elevated indirect HA titers and local reactions resembling delayed hypersensitivity, were striking in individuals whose immunity had been primed by previous inoculations with relatively small doses of live, attenuated, but invasive Y. pestis E.V.76 daughter strains. A freeze-dried, USP (Cutter) vaccine suspended in cryoprotective chemicals (assayed and proven potent in terms of its capacity, as compared with a standard vaccine, to protect mice and guinea pigs against a standard challenge with virulent Y. pestis) proved to be only moderately antigenic in man. As a booster antigen, it stimulated a significant humoral response. Antigenic and immunogenic inadequacies of killed plague vaccines were overcome by reinoculation at three- to six-month intervals.