Evidence for the presence of suppressor cells in B6AF1 mice bearing long-term C3H skin grafts after multiple blood transfusions and antilymphocyte serum (ALS) treatment is described. Transfer of spleen cells from these mice into secondary B6AF1 recipients given ALS and C3H skin grafts resulted in marked prolongation of graft survival. The spleen cells were also capable of specifically inhibiting the proliferative response of normal B6AF1 responders to C3H stimulator cells in coculture mixed lymphocyte culture (MLC) experiments.