SummaryUrethane, administered intraperitoneally in subanesthetic dose, 0.75 mg per g of body weight per day in leukemic mice transplanted with leukemic cells of the 32nd to 34th transfer generations of an acute lymphoid leukemia, produces the following profound palliative effects: (1) The blood leukocytes are maintained at leukopenic levels. (2) The immature lymphoblasts are main tained at significantly lower levels than control leukemics and in some cases disappear entirely from the peripheral blood. (3) Hemo globin values remain at higher levels in the urethane-treated animals throughout treatment.(4) Local subcutaneous growth of the tumor mass is retarded. Some infiltration of lymphoblasts into spleen, thymus and lymph nodes occurs, however this is not of the characteristic massive and diffuse type. Lymphoblast destruction is evident. (5) A statistically significant greater life expectancy in urethane-treated leukemics is obtained.