The ability of CD4+ and CD8+ T cells to adoptively immunize mice against Chlamydia trachomatis infection of the mouse genital tract was studied. Adoptive transfer experiments were performed with splenic CD4+ or CD8+ T cells obtained from mice following resolution of a primary genital tract infection and after a secondary chlamydial challenge. The results show that donor CD4+ T cells, but not CD8+ T cells, obtained from mice following resolution of a primary infection or after secondary challenge were effective in transferring significant antichlamydial immunity to the genital tracts of naive animals. The lymphokine profiles in the culture supernatants of proliferating Chlamydia-specific CD4+ T cells obtained from mice following resolution of a primary infection and after secondary challenge were assayed by an enzyme-linked immunoadsorbent assay. Protective CD4+ T cells restimulated in vitro secreted interleukin 2, gamma interferon, and interleukin 6, lymphokine profiles characteristic of both Th1- and Th2-like responses. Resting CD4+ T cells obtained from mice 4 months following resolution of a primary infection were also capable of conferring significant levels of adoptive protective immunity to naive mice. These findings support an important role for CD4+ T cells in acquired immunity to chlamydial infection of the genital tract and indicate that protective CD4+ immune responses in this model are relatively long lived.