Abstract In the erythrocytes and liver of a patient with hereditary non-spherocytic hemolytic anemia and increased serum aminotransferases, almost complete deficiency of L-type pyruvate kinase was detected. The parents of the patient are second cousins and the pyruvate kinase activity in their erythrocytes was decreased to about half normal values. Pyruvate kinase from the patient is characterized by extreme lability. Pyruvate kinase from the parents' red cells showed no molecular abnormalities. No cross-reactive material could be precipitated with a monospeoific antibody raised against L-type pyruvate kinase in the patient's erythrocytes. In the red cells of the parents a decreased amount of cross-reactive material against pyruvate kinase antibodies was found, indicating that the lowered pyruvate kinase activity in the erythrocytes of the parents is caused by a decreased level of the pyruvate kinase protein. In the liver of the patient no L-type pyruvate kinase activity and no immunologically recognizable L-type pyruvate kinase could be detected. The increased lability of the enzyme protein may explain the low residual activity. However, this decreased activity is shown to be sufficient to perform a normal glycolytic flux resulting only in moderate clinical expression.