Pyruvate kinase (PK) from four patients with moderate to severe congenital non-spherocytic haemolytic anaemia was characterized by methods recommended by the ICSH. The possibility that two of the patients are true homozygotes cannot be ruled out, while the other two apparently represent double heterozygotes. All but one had levels of PK activity between 44 and 65% of normal. The variant enzymes were designed 'PK Pontos', 'PK Macedonia', 'PK Athens' and 'PK Larisa'. Multiple physicochemical as well as kinetic aberrations were detected in the above variants. Their altered kinetic behaviour is discussed in terms of the concerted transition model for allosteric enzymes and their abnormal properties are compared with other known variants, while it is also attempted to correlate them with possible mechanisms resulting in chronic haemolytic anaemia.