DA rat sarcomas P, and P, were induced by dimethylbenz (a) anthracene. A tumor-specific immune response of DA rats against P1-tumor cells could be demonstrated at the humoral and cellular level. DA anti-P1 antibodies were purified on fixed Prtumor cells and used as autoimmunogen in DA rats for the production of antiidiotypic antibodies. Such antiidiotypic antibodies could be demonstrated by using a solid-phase radioimmunoassay and by their ability to induce secondary type of DA anti-P1 responses in vitro. In addition, such antibodies were able to induce cytotoxic T lymphocytes capable of eliminating P1-tumor cells but not control tumor cells. In some of the auto-immunized DA rats enhanced P1-tumor growth could be observed, indicating that the anti-idiotypic immune response had led to a selective ablation of idiotypic, potential anti-P1 reactive T cells.