We have produced a contrasuppressor T cell hybridoma which has positive effects on multiple forms of cell-mediated immunity. First of all, it protects the adoptive transfer of contact sensitivity from suppressor cells and factors. In addition, TcsF modifies the response to normally tolerogenic administrations of hapten, leading mice to develop contact sensitivity and CTL activity instead of tolerance. Most relevant to this conference, mice which have been both treated with AF5.C6 TcsF and painted with TNCB resist challenge with highly malignant TNP-modified tumors. These experiments suggest a decisive role for contrasuppression in tumor rejection.