To study the immunological protective system against rickettsial infection, a T-cell line specific for Rickettsia tsutsugamushi antigen was established by long-term culture of splenocytes from mice immunized with live Gilliam strain R. tsutsugamushi and then propagated in the presence of homologous rickettsial antigen and syngeneic filler cells. The characteristics of the T-cell line and its capacity to induce antirickettsial protection in vivo were studied. Flow cytometric analysis demonstrated that the T-cell line showed the phenotype Thy-1.2+ L3T4+ Lyt-2-, suggestive of helper T cells. In a lymphocyte proliferation assay, this cell line showed a specific response to Gilliam antigen, partial cross-reactivity to Karp antigen, but no response to Kato antigen. The proliferative response of this T-cell line was filler cell dependent, and genetic restriction was observed between the T-cell line and filler cells. The T-cell line produced gamma interferon, one of the macrophage-activating factors, in cultures with specific antigen and was able to adoptively mediate antirickettsial protection in vivo. The data presented here suggest that antigen-specific helper T cells play an important role in protection against rickettsial infection.