Delayed-type hypersensitivity (DTH) against histocompatibility (H) antigens in mice, which normally arises after s.c. immunization, can be prevented by i.v. preimmunization with irradiated spleen cells carrying the relevant H antigens. We have previously shown that during the first week after i.v. preimmunization the nonresponsiveness is due to suppressor T lymphocytes. The induced state of nonresponsiveness, however, is long-lasting. In this study we investigated whether this long-lasting state of nonresponsiveness of DTH is associated with suppressor T lymphocytes or is caused by inactivation of the relevant clones of alloreactive T cells. This was done by parabiosis of nonresponsive and naive mice and by transfer of thoracic duct lymphocytes from nonresponsive mice, harvested at various intervals after the i.v. immunization. At a long interval after the i.v. immunization, the state of nonresponsiveness could still be transferred to syngeneic naive mice by parabiosis, as well as by transfer of thoracic duct lymphocytes. Selective elimination of the T cells from the latter by treatment with anti-Thy-1.2 plus complement prevented the transfer of the state of nonresponsiveness, indicating that suppressor T cells were involved.