The capacity of a purified preparation of Legionella pneumophila flagella (FLA) to induce protective immune responses was studied in an A/J mouse model. Animals immunized with FLA promptly mounted an anti-FLA antibody response and also developed a strong activation of both innate and adaptive cell-mediated immunity, as shown by an early release of pro-inflammatory cytokines in the peritoneal cavity, and by a positive cutaneous delayed-type hypersensitivity reaction and in vitro splenic lymphocyte proliferation in response to FLA antigens. Mice treated with FLA either i.v. or i.p. also survived (100% rate) a lethal i.p. challenge with L. pneumophila. Protection induced by FLA lasted for at least 30 days after treatment, but less than 60, and was effective against the challenge with different serogroups of L. pneumophila. Resistance conferred by FLA immunization could be partially transferred to naïve animals by the adoptive transfer of immune splenocytes but not by passive immunization with anti-FLA iperimmune sera. The capacity to induce protective immunity was specifically attributable to flagellar components, as demonstrated by the lack of protection in mice immunized with a sham flagella preparation from a non-flagellated bacterial strain or with protease-digested FLA. In addition, heat-denatured FLA was inactive, suggesting loss of immunogenicity following denaturation. The present study provides evidence that L. pneumophila flagellum is strongly immunogenic and capable to stimulate, without adjuvants, early natural and acquired, T-cell-mediated immune responses and to induce significant protection against a lethal bacterial challenge in A/J mice. Antigenic characterization of this bacterial organelle and elucidation of mechanisms underlying flagella-induced protection would be of great value in understanding the immunopathogenesis of the disease and in developing possible therapeutic strategies for human legionellosis.