The functional properties of granulocytes in a diabetic patient deficient in myeloperoxidase (MPO) were compared with those of granulocytes in healthy subjects. The granulocytes of this patient had normal phagocytic activity. The microbicidal activity of the granulocytes was partially diminished with regard to Staphylococcus aureus and was almost nil with regard to Candida albicans. Fungicidal activity of normal granulocytes was shown to be impaired during the in vitro artificial hyperglycemic condition. The relationship among diabetes mellitus, MPO deficiency, and serious C. albicans infection was examined. Genetic investigation was carried out in 28 members of the proband's family. In close relatives of the patient, MPO values were found to be diminshed to a greater or lesser degree, thus suggesting variable expressivity of the heterozygote state of MPO deficiency.