Panels of cloned T-cell lines were derived from the autologous mixed lymphocyte reactions of NZB and C58 mice. These clones were all Thy 1+. In addition, various clones expressed appropriate Ia, Lyt 1 and/or Lyt 2 antigenic specificities. None of these clones produced the lymphokines IL-2, CSF or AMLR-helper factor. The clones suppressed fresh syngeneic AMLR and MLR responses when added at low cell numbers at the initiation of culture. This suppression was not abrogated by treatment with mitomycin c or reversed by the addition of a source of T-cell growth factor. The mechanism of suppression was not cytotoxicity, as the clones were non-cytotoxic for either syngeneic or allogeneic cells. Many of the clones appeared to require the presence of Lyt 2+ cells in the MLR responding population to suppress, and therefore can be classified as T-suppressor inducers. Two clones did not require the Lyt 2+ subset to suppress the MLR, and are therefore T-suppressor effectors.