Reversible haemolytic anaemia associated with decreased red cell half-life and reticulocytosis was studied in 10 patients with Zieve's syndrome. Since the underlying cause of the red cell destruction is as yet unknown, we determined the critical metabolic functions of the red cells in order to define the assumed intracorpuscular defect causing haemolysis. The glucose metabolizing enzymes had normal or raised values. - In view of the diminished ATP and raised 2,3-diphosphoglycerate (2.3 DPG) levels - a combination which suggests a pyruvate kinase (PK) deficiency - additional procedures were carried out in order to detect an abnormal activity of the red cell PK. Studies of biochemical properties of PK such as thermostability, Michaelis-Menten constants, and activation and inhibition tests brought results markedly deviating from the norm.-Fractions containing old cells particularly disclosed PK instability. A defective red cell matabolism resulted which was measurable through ATP-instability, altered glucose utilization and lactate production. - Experimental cell aging procedures led to a markedly decreased red cell metabolism. These assays revealed that mutations of PK-control mechanisms might be involved as factor triggering haemolytic anaemia of Zieve's syndrome.