Biological properties of suppressor cells mediating second suppressive state (S-SupS) of IFN-gamma responsiveness in mice following thermal injury were investigated by their ability to inhibit [3H]thymidine incorporation by allogeneic cell-stimulated mononuclear cells (MNC) prepared from spleens of unburned normal mice (N-mice). Kinetic studies indicated that the suppressor cells appeared at approximately 15 days post-burn and persisted until 40 days after thermal injury. This cell population from the spleens of thermally injured mice (TI-mice) was inactivated by anti-Thy 1.2 monoclonal antibody (mAb) plus complement (C). Treatment with antimouse immunoglobulin and anti-Lyt 1.2 mAb followed by C failed to abrogate the activity. However, it could be eliminated by treatment with anti-Lyt 2.2 mAb and C. In addition, when Thy 1+ or Lyt 2+ T cells were depleted from spleen cells containing suppressor cell activities by treatment with mAbs and C, the suppressor activity for IFN-gamma production by N-mice splenic MNC stimulated with concanavalin A was also eliminated. However, no alteration in the suppression of IFN production was observed when this splenic cell population from TI-mice was treated with anti-Lyt 1.2 mAb and anti-mouse Ig followed by C. Since it has been reported [1] that the suppressor cell in spleens of TI-mice could neither be removed by plastic adherence nor by carbonyl-iron treatment, this secondarily-generated suppressor cell was concluded to be a T cell possessing the Lyt 1-2+ phenotype. The present report describes the demonstration, following a generation of suppressor macrophages [2], of suppressor Lyt 2+ T cells spleen of mice late after thermal injury.