The behaviour of committed CFU-GM bone marrow cells was investigated in leukaemic and preleukaemic AKR/J mice. The number of bone marrow CFU-GM decreased in the leukaemic stage of spontaneous AKR/J leukaemia. The addition of AKR/J leukaemic cells inhibited CFU-GM growth from normal target bone marrow. The bone marrow CFU-GM growth was also reduced in late preleukaemic AKR/J mice, whereas no modification in the CFU-GM number in early preleukaemic animals was found.