Four people from the same family with red cell pyruvate kinase (PK) hyperactivity are described. These people displayed low 2,3-diphosphoglycerate (2,3-DPG) and high adenosine triphosphate (ATP) levels. In vitro metabolism studies of their red cells showed the relationship between the PK activity, low 2,3-DPG, and high ATP levels. The PK electrophoretic pattern of these subjects was abnormal by the presence of several additional bands; one of them migrated like PKM2. PKR from these people was thermounstable and M2-like PK was identical to PKM2 for immunologic reactivity and KO, 5s for phosphoenolpyruvate.