W e have examined the capacity of the major cytoplasmic membrane protein (MCMP) of Legionella pneumophila, a genus common antigen and member of the hsp 60 family of heat shock proteins, to induce protective immunity in a guinea pig model of Legionnaires' disease. We purified MCMP to homogeneity from L. pneumophila by buffer extraction, ion-exchange chromatography, and molecular sieve chromatography. Guinea pigs immunized with MCMP developed a strong cell-mediated immune response to the immunogen manifest by marked cutaneous delayed-type hypersensitivity. Guinea pigs immunized with MCMP and then challenged with a lethal aerosol dose of L. pneumophila exhibited a high level of protective immunity. Altogether, in four independent experiments, 55 of 64 (86%) animals immunized three times with 0.6-40 micrograms MCMP including 11 of 11 (100%) animals immunized three times with 40 micrograms MCMP survived aerosol challenge with L. pneumophila compared with 1 of 29 (3%) sham-immunized control animals (P < 0.0001, Cochran-Mantel-Haenszel X2 statistic for pooled data). To our knowledge, MCMP is the first member of the hsp 60 family of proteins shown to induce protective immunity to a microbial pathogen. MCMP has potential as a vaccine against Legionnaires' disease. Since MCMP is a genus common antigen, vaccination with a combination of MCMPs derived from different Legionella species has the potential of inducing protective immunity against all the major Legionella species causing human disease.