The activation of some subsets of T cells requires the recognition of antigen in association with self Ia determinants. It is not clear, however, whether this is also necessary for the induction of unresponsiveness and the active suppression of hapten-specific T cells. We have studied the regulation of the delayed-type hypersensitivity (DTH) response in mice. Subcutaneous (s.c.) injection of haptenated syngeneic cells primarily activates T helper cells, while intravenous (i.v.) injection results in unresponsiveness and the activation of T suppressor cells. Presentation of antigen by an I-A-positive antigen-presenting cell (APC) seems to be critical for T helper cell activation. We now show that the activation of first-order T suppressor cells requires the presentation of hapten on an I-J-positive APC. However, i.v. injection of I-J-depleted haptenated cells also results in hepten-specific unresponsiveness. This non-transferable T-cell tolerance requires the presence of an I-A-positive APC and is genetically restricted. These results suggest that distinct modes of antigen presentation and administration are required for immunity, suppression and non-transferable tolerance.