We have examined the capacity of the major secretory protein (MSP) of Legionella pneumophila to induce humoral, cell-mediated, and protective immunity in a guinea pig model of Legionnaires' disease. MSP was purified to homogeneity by ammonium sulfate precipitation, molecular sieve chromatography, and ion-exchange chromatography. The purified MSP was nonlethal and nontoxic to guinea pigs upon subcutaneous administration. Guinea pigs immunized with a sublethal dose of aerosolized L. pneumophila or a subcutaneous dose of MSP developed a strong cell-mediated immune response to MSP. Such guinea pigs exhibited marked splenic lymphocyte proliferation and cutaneous delayed-type hypersensitivity to MSP in comparison with control animals. Guinea pigs immunized with MSP also developed a strong humoral immune response to MSP, as assayed by an ELISA. The median reciprocal antibody titer was 362 (range 45 to greater than 2,048) for immunized animals compared with less than 8 for controls. In contrast, guinea pigs immunized with a sublethal dose of L. pneumophila failed to develop anti-MSP antibody. Guinea pigs immunized with MSP and then challenged with a lethal aerosol dose of L. pneumophila exhibited highly significant protective immunity in each of five consecutive experiments. MSP induced protective immunity in dose-dependent fashion (40 greater than 10 greater than 2.5 greater than 0.6 micrograms MSP); vaccination with two doses of as little as 2.5 micrograms MSP induced significant protective immunity (p = 0.01, Fisher's Exact Test, two-tailed). Altogether, 21 (81%) of 26 animals immunized with 40 micrograms MSP survived challenge compared with 0 (0%) of 26 sham-immunized control animals (p = 7 x 10(-10), Fisher's Exact Test, two-tailed). MSP-immunized but not control guinea pigs were able to limit L. pneumophila multiplication in their lungs. This study demonstrates that (a) guinea pigs sublethally infected with L. pneumophila develop a strong cell-mediated immune response to MSP; (b) guinea pigs immunized with MSP develop a strong humoral and cell-mediated immune response to MSP; (c) guinea pigs immunized with MSP develop a very high level of protective immunity to lethal aerosol challenge with L. pneumophila; and (d) MSP-immunized animals are able to limit L. pneumophila multiplication in their lungs. MSP, an extracellular protein of an intracellular pathogen, has potential as a vaccine for the prevention of Legionnaires' disease. Secretory molecules of other intracellular pathogens may also have vaccine potential.