A complete lack of myeloperoxidase (MPO) was demonstrated in a boy suffering from acute myeloic leukemia during the acute phase of the disease and after a remission was achieved. A partial defect of MPO was demonstrated in the patient's father, no further abnormalities were seen in other members of the family. The fine structure of the patient's neutrophils and monocytes appeared normal, no activity of MPO was demonstrated on the fine structural level. In the father's neutrophils transitional forms between cells exhibiting a normal MPO activity and those without activity were demonstrated. The neutrophil bactericidal activity was strongly inhibited in the patient and decreased in his father. Normal values were found in: NBT test, chemotaxis, serum-dependent phagocytosis, number of B and T lymphocytes, serum immunoglobulins, and complement. A possible connection between MPO deficiency and leukemia is discussed.