Superoxide dismutases with molecular weight of 65-67 and 200 kD have been identified in Yersinia pestis EV cells. These enzymes have some differences in antigenic specificity, resistance to heating and inhibitors (cyanides and hydrogen peroxide). The activity of Y. pestis superoxide dismutase is a thermoinducible sign unassociated with the presence of residual plasmids of the bacilli. Superoxide dismutase can be released from the cells into liquid culture medium. Patients inoculated with live plaque vaccine develop a humoral response to high molecular-weight superoxide dismutase. Immunization of guinea pigs with purified high molecular-weight superoxide dismutase ensures their specific protection after subcutaneous injection.