Both B- and T-cell immunogenicity of a chlamydial 75-kDa protein was analyzed by using 131 partially overlapped decapeptide homologs of the 75-kDa protein from Chlamydia trachomatis serovar L2. Six rabbit antiserum specimens raised with serovars B, C, and L2 were used to assay the antibody reactivities of the decapeptides. Seventy-five of the 131 decapeptides were recognized by at least one antiserum specimen, and two peptides were found to be immunodominant and surface accessible on native organisms. The same set of decapeptides were cleaved from the pins and tested for their T-cell-stimulating activity in an in vitro proliferation assay. A single decapeptide was able to stimulate proliferation of chlamydial antigen-primed lymph node T cells from BALB/c mice.