Decreased actinomycin D binding was observed in the marrow cells of some patients with chronic myeloid leukaemia. Blast uptake was still reduced in 2 cases of blast crisis and in acute myeloblastic leukaemia. However, the differentiating cells in blast crisis, i.e., those that had passed the blast stage, had exactly the same actinomycin D binding values as normal myeloid cells. After 0.5 mg/kg methotrexate, increased actinomycin D binding on the part of the entire leukaemic population was observed in one case of acute myeloblastic leukaemia. These findings suggest differences in chromatin behaviour and genetic activity in myeloid cells in the different stages of myeloid leukaemia.