A procedure for the complement-fixation test with plague Fraction I as antigen and specific high-titered antiplague (by preference anti-Fraction I) serum as antiserum has been described. Examples of the use of the method are given: the determination of Fraction I plague antigen in the bacteria and in tissue extracts of carcasses of mice and guinea pigs that had died of experimental plague. Antibodies to Fraction I were detected in convalescent human sera, in sera of guinea pigs immunized with various living avirulent plague organisms, and of rabbits and monkeys immunized with Fraction I. The titers obtained are in direct agreement with values determined by precipitation, agglutination, immunization and passive mouse-protection tests. The complement-fixation tests, besides being a valuable addition to research plague studies, also acts as a liaison test for evaluation of plague vaccines, estimation of degree of immunity, determination of potency of antiplague sera, confirmation of recent infection with Pasteurella pestis and help in the positive identification of plague bacilli.