Autologous bone marrow transplantation is a new technique in which some of the patient's bone marrow is removed and frozen to be reinjected later after heavy chemotherapy and/or total irradiation. Applied to solid tumours, this technique makes it possible to use tumoricidal agents to a degree never achieved before. In acute leukaemias bone marrow can be taken during complete remission, cleared in vitro of contaminating blast cells and reinjected after heavy chemotherapy and/or total irradiation have been given to consolidate the remission. The preliminary results of international trials indicate that this new approach to the treatment of acute leukaemias should substantially prolong the first complete remission and most probably increase the cure rate. The different methods used for in vitro depuration of leukaemic bone marrows (e.g. chemotherapy, monoclonal antibodies, immunotoxins) are described.