Fetal and adult erythrocyte characteristics were studied serially in a 30-mo-old female with juvenile chronic myelocytic leukemia. On presentation the erythrocytes exhibited predominantly fetal characteristics as indicated by 69% hemoglobin F (HbF), 1.1% hemoglobin A2 (HbA2), absent I antigen, and fetal levels of the erythrocyte enzymes, carbonic anhydrase I and II, glucose-6-phosphate dehydrogenase, hexokinase, pyruvate kinase, and lactate dehydrogenase; 100% of the erythrocytes present contained HbF. However, Orskov-Jacobs- Stewart hemolysis demonstrated that at least one adult characteristic was present. Seven months later HbF was 17%; I antigen and carbonic anhydrase I had increased to adult levels. The number of cells containing HbF had decreased to 30%. Further studies indicated that at least three new populations of red cells were present after 7 mo which had not previously been detected. Two of these populations exhibited a mixture of both fetal and adult characteristics. Such findings suggested that an ongoing disturbance of regulatory mechanisms was responsible for the variable expression of fetal versus adult erythrocyte characteristics.