Antisera against the C57B1 (H-2b) mouse lymphoma, EL4, were prepared in rabbits. After absorption with mouse liver, red cells and thymocytes, the antisera appeared to be cytotoxic for a subpopulation of peripheral T cells. The absorbed antisera blocked the immunosuppressive function observed when spleen cells from mice undergoing a graft versus host reaction were added to cells responding in vitro to sheep erythrocytes. This antiserum was unreactive against the cytotoxic cells also induced in this system. These results substantiate our previous finding that these antisera are specific for T-suppressor as opposed to T-helper or cytotoxic lymphocyte subpopulations.