Bone marrow erythroid progenitor cells were examined from 50 cases of acute leukaemia and from 20 normal subjects using an in vitro semisolid culture method. Numbers of both primitive erythroid progenitor cells (BFU-e) and later-stage erythroid progenitor cells (CFU-e) were remarkably depressed in patient with acute leukaemia in active phase. However, both BFU-e and CFU-e recovered to within normal range when the patients achieved remission. Peripheral blood BFU-e of children with acute lymphocytic leukaemia in remission were also examined and found to have values not significantly different from those of control subjects. There was no distinct correlation between the numbers of erythroid bursts or colonies and the duration of remission in patients with acute leukaemia in remission. The reduction of BFU-e and CFU-e in active acute leukaemia suggests the involvement of erythropoietic progenitors in the pathophysiology of this type of leukaemia.