Investigations on the activity and the kinetic behavior of red cell pyruvate kinase in the erythrocyte membrane and hemolysate of 6 patients with pyruvate kinase (PK) deficiency are presented and compared with those of normal individuals. Hemolysate: The PK in the hemolysate of normal individuals showed the well-known allosteric properties, reflected by sigmoidal reaction kinetics (Hill-coefficient > 1). In contrast to these findings the enzymes of 4 of the patients which had not been splenectomised and which were all in satisfactory condition showed between 15 and 35% of the PK activity of normal individuals in the hemolysates, whereas in the case of the 2 splenectomised patients only 5 and 15 % of this activity could be measured. All of the patients’ enzymes showed non allosteric (hyperbolic) reaction kinetics (Hill-coefficient ~ 1.0). Red cell membranes: Our studies confirmed earlier reports that not only the hemolysate of the patients, but also their red cell membranes (‘ghosts’) exhibit PK activity. In normal individuals the activity of the membrane-localised PK is about 5% of the cytoplasmatic one. In case of the membrane-localised enzyme of all patients - irrespective if they were splenectomised or not - the activity is decreased to 30-50%, compared to the activity of ghosts of the control persons. Reaction kinetic studies on the PK isolated from red cell membranes from normal individuals as well as from all 6 patients exhibited hyperbolic Michaelis-Menten kinetics with Hill coefficients around 1.0.