The paper contains the results of three cytochemical reactions used for detection of myeloid differentiation (peroxidase, sudanophilia, chloroacetate esterase) in neutrophil segments of the peripheral blood stream in 107 patients with acute myeloid or lymphatic leukaemia. Enzymatically deficient segments were detected in 23 (34.8%) patients with acute myeloid leukaemia. They were not found in any patients lymphatic leukaemia not in healthy subjects. The concurrent deficit of all three reactions was found in 69% of the cases with defective neutrophil segments. However, also isolated affection of any of these reactions was found. In acute leukaemias, not differentiated from the cytochemical and immunophenotypical aspect, we may consider simple evidence of the presence of the mentioned abnormal neutrophil segments in peripheral blood as a highly probable sign of myeloid differentiation of acute leukaemia.