We studied a family with a partial myeloperoxidase deficiency. The myeloperoxidase in the neutrophils and monocytes of the parents and their two sons had normal spectral properties (determined optically and by EPR). Enzymic characteristics (oxidation of iodide) were indistinguishable from those of normal myeloperoxidase; moreover, immunological identity between the myeloperoxidase in the leukocytes of the family members and normal myeloperoxidase was found. No differences in heat stability were observed. The neutrophils and monocytes of the sons contained 9% to 18% of the myeloperoxidase content of normal cells; the neutrophils and monocytes of the parents contained 45% to 58%. These data suggest either that the parents are heterozygous and the sons homozygous for hereditary partial myeloperoxidase deficiency or that each parent is heterozygous for a different type of myeloperoxidase deficiency and the sons combine both deficiencies. The oxidative metabolism of the neutrophils during phagocytosis was not affected by the myeloperoxidase deficiency. The killing of Staphylococcus aureus was apparently normal. The perforation of Escherichia coli by the neutrophils of the sons, however, was retarded in comparison with normal neutrophils.