A new mutant pyruvate kinase (ATP: pyruvate phosphotransferase, EC 2.7.1.40) from human erythrocytes is described. The mutant enzyme shows an increased thermolability, a decreased Km value for the substrate phosphoenolpyruvate and a loss of allosteric properties during the lifespan of the erythrocytes. By comparing the previous obtained data from other patients it seems that there can be distinguished at least three types of pyruvate kinase deficiencies