An alkylating agent having a selective immunosuppressive effect on bone marrow. It has been used in the palliative treatment of chronic myeloid leukemia (MYELOID leukemia, CHRONIC), but although symptomatic relief is provided, no permanent remission is brought about. According to the Fourth Annual Report on carcinogens (NTP 85-002, 1985), busulfan is listed as a known carcinogen.