Sonicates from five cultivable treponemes were used as antigens in delayed hypersensitivity tests and macrophage inhibition assays. Immunodiffusion analysis showed that the sonicate comprised two major antigenic components which were not distinguishable in the skin tests. The sonicate antigens elicited significant cell-mediated immunity in guinea-pigs. Treponema refringens biotype Nichols proved to induce the strongest delayed response. Delayed skin hypersensitivity to the antigens of Treponema pallidum was found in eight rabbits without orchitis, but not in six rabbits with T. pallidum orchitis. In contrast, the rabbits with syphilitic orchitis gave the strongest reactions with the non-pathogenic spirochetes. In terms of the cell-mediated immunity responses, Treponema phagedenis Reiter was found to be related to T. phagedenis Kazan 4 and Treponema denticola. Treponema scoliodontum was related to T. phagedenis Kazan 5, and T. refringens biotypes Nichols and refringens. The antigens of T. pallidum had the closest relationship to T. refringens biotypes refringens and Nichols ,T. phagedenis biotype Reiter, and T. scoliodontum. It was also demonstrated that some three of 12 human syphilitic sera reacted with the antigens of T. pallidum but not with control sera.