Summary Seven new red-cell pyruvate kinase (PK) variants were characterized by the methods recently recommended by the International Committee for Standardization in Haematology. The cases were all true homozygote as evidenced by consanguineous marriages of the parents; all are Japanese. These variants were designated as PK Tokyo, PK Nagasaki, PK Sapporo, PK Maebashi, PK Itabashi, PK Fukushima and PK Aizu, respectively. Low substrate affinity (high K0.5S for phosphoenolpyruvate) and thermal instability appear to play major roles in causing defective enzyme function, resulting in chronic haemolytic anaemia. Product inhibition of PK by ATP may also play an additional role in causing haemolysis in more than half the cases.