In a patient with nonspherocytic hemolytic anemia, a hexokinase deficiency was detected in the red cells (residual activity about 25% of normal) and in blood platelets (20%-35% of normal activity). Although the total hexokinase activity in lymphocytes was normal, the amount of hexokinase type I was decreased to about 50% of normal. However, the deficiency was compensated for by the appearance of type III hexokinase. Compartmentation studies with controlled digitonin- induced cell lysis showed that this type III enzyme was localized in the cytosol, while almost all hexokinase activity in normal lymphocytes is particulate. No abnormal lymphocyte functions could be detected. The patient was homozygous for the defect. The parents and three of five sibs of the patient were apparently heterozygous with residual activities of 50%-67% of normal in their red cells, but did not show any clinical signs of hexokinase deficiency. The variant enzyme had a slightly decreased affinity for MgATP2- and a strongly increased inhibition constant for glucose-1,6-P2. Affinity for glucose, heat stability, and pH optimum were normal. In the electrophoretic pattern of red cell hexokinase, only one subtype of hexokinase I could be detected, while in normal red cells, at least three subtypes are present. In the heterozygous individuals, no enzymatic abnormalities could be detected, except for an aberration in the electropherogram of one sib.