Lymphoid cells from ALS-treated (C57BL/6 x A/J)F1 (B6AF1) mice bearing enhanced C3H/He grafts were assayed for their ability to suppress the response to C3H/He grafts after transfer to syngeneic B6AF1 recipients. Cells were transferred from ALS-treated B6AF1 mice that had received either a C3H/He graft alone, C3H/He marrow alone, or both a graft and marrow. Suppressor cells appeared in the spleens of ALS-treated B6AF1 mice that had received either a graft alone or both graft and marrow as early as day +13. They persisted only in the spleens of mice that had received both a graft and marrow, i.e., mice whose grafts showed significant prolongation. Suppressor cells did not appear in the lymph nodes of mice bearing enhanced grafts until day +42. Thymocytes and bone marrow cells were unable to transfer unresponsiveness. The cells which transferred unresponsiveness were specific for the graft donor strain as they did not transfer unresponsiveness to third-party grafts. The ability of cells to transfer suppression was abrogated by treatment with anti-theta serum.