A method for isolating pure reticulocytes from leukocyte-depleted blood of normal persons is presented. The separation was achieved using an immunomagnetic technique. A monoclonal mouse antibody against human transferrin receptor was bound to magnetic beads conjugated with sheep antimouse antibody. The recovery of reticulocytes from peripheral blood was 15% to 42%. Blood used for isolation of reticulocytes could be stored for 4 days at 22 degrees C without altering the yield of reticulocytes. At 37 degrees C incubation, the reticulocytes matured rapidly and the transferrin receptor was found to have a half-life of 16 hours. The activity of several enzymes and the amount of creatine and hemoglobin A1C were measured both in the reticulocytes and peripheral blood. Of the enzymes, porphobilinogen deaminase had the best discriminatory power with a ratio of 8.8 between reticulocytes and peripheral red blood cells. The ratio for creatine was 16.7. The ability to isolate pure human reticulocytes, released after normal erythropoiesis, will offer new possibilities in the study of these cells.