Di-iodobenzotepa (DIB) was given per os daily for 3 days in a dose of 25 mg/kg to CBA mice. A decrease in the number of nucleated cells in the thymus by 74% and in the bone marrow by 29% was observed. Experiments with transplantation of lymphocytes from mice treated with DIB into intact or lethally irradiated (CBAxC57BL/6J)F1 mice showed that DIB has no effect on the helper activity of T-lymphocytes but depresses the functional activity of B-lymphocytes and t-lymphocytes, inducing the graft versus host reaction, and of T-suppressor cells. © 1980 Plenum Publishing Corporation.