Haemoglobin fractions and 16 enzymatic activities of red cells of a patient with juvenile chronic myeloid leukaemia are compared to normal, to comparably reticulocyte-rich, non-neonatal and to fetal red cells. The activities of hexoki-nase, triosephosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase, mono-phosphoglyceromutase, enolase and glucose-6-phosphate dehydrogenase are significantly increased in fetal red cells beyond the activities of cell populations with comparable reticulocytosis. The activities of these enzymes are also increased in the patient’s erythrocytes. Together with a haemoglobin F concentration of 54% and a concentration of haemoglobin Bart’s of l% these variations reflect the fetal nature of the red cells. Simultaneously, signs of dyserythropoiesis are found in the red cells of the patient: a very high activity of hexokinase and a low pyruvate kinase activity.