S-phase fraction of granulocyte-macrophage colony forming cells (CFU-GM) was assessed by cytosine arabinoside suicide technique in five normal volunteers and eight patients with acute myeloblastic leukaemia in complete remission after the cessation of polychemotherapy. An increased proliferation of CFU-GM (S-phase-fraction: 55-80%) was found in bone marrow of patient in complete remission up to the 27th day after polychemotherapy. Reduction of this increased proliferation was seen at day 30 after the cessation of polychemotherapy (S-phase fraction: 22 and 33%). Because of increased proliferation of CFU-GM in the early phase after polychemotherapy cytostatic drugs of the antimetabolite-type seem to be unsuitable for purging bone marrow in remission.