Spleen cells from thymectomized antilymphocyte serum (ALS)-treated B6AF1 mice bearing enhanced C3H/He grafts after the infection of C3H/He marrow were assayed for their ability to suppress the response to C3H/He grafts after transfer to syngeneic B6AF1 recipients. Cells were transferred from thymectomized ALS-treated B6AF1 mice that had received either a C3H/He graft alone, C3H/He marrow alone, or both a graft and marrow. Cells were removed from donors and transferred at either day +13, +42, +62, +100, or +150. Spleen cells from thymectomized mice were unable to transfer unresponsiveness regardless of donor treatment or time of transfer.