The role of suppressor cells in the induction of unresponsiveness to skin allografts will be studied in two thymectomized models. In the first model, adult mice are thymectomized before treatment with ALS and the injection of donor strain bone marrow. These mice are not cellular chimeras, but 40 percent maintain their skin grafts permanently. In the second model, adult mice are thymectomized before treatment with ALS and the injection of large numbers of hybrid lymphoid cells. These mice are cellular chimeras and maintain their grafts permanently. The presence of suppressor cells will be assayed by the spleen transfer assay. The immune status of ALS treated, marrow injected mice will be studied by various in vitro assays. Lymphoid cells from mice that have been treated with either ALS alone, ALS and a skin graft. ALS and marrow or ALS, graft and marrow will be assayed in mixed lymphocyte cultures and in cytotoxic assays. The efficacy of third party blood transfusions in inducing unresponsiveness to skin allografts will also be studied in ALS treated mice.