CBA mice were treated before skin grafting with a course of either the methylhydrazine derivative Ro 4-6824 or cyclophosphamide, combined with donor type lymphoid cells. No further immunosuppressive treatment was applied after the grafting. Marked prolongation of graft survival and specific tolerance to (CBAxA)F1 and A skin homografts could be induced with Ro 4-6824, but not with cyclophosphamide. Tumors occurred in two tolerant mice after 10 months. Besides by administration of donor lymphoid cells, the effect of Ro 4-6824 pretreatment is also potentiated by heterologous antilymphocytic serum (ALS) and by thymectomy. The mice treated with Ro 4-6824 showed more profoud depletion of lymphoid tissue in the lymph nodes than after ALS. In addition, marked atrophy of the thymus occurred. In the spleen, Ro 4-6824 prevented the plasma cell response to ALS.