The phagocytic ability and the proliferative activeity of leukaemic blast cells were investigated in 24 patients with actue leukaemia. A certain phagocytic activity was found in most cases of acute leukaemia. The highest phagocytic activity occurred in monocytic leukaemia. Blast cell populations with a relative high phagocytic activity showed a low proliferative activity and only a few cells in DNA synthesis. The blastcells with phagocytosis are more mature cells. We got the impression that they areresting cells and possibly end cells