The present study evaluates the properties of the reticulocytes produced in healthy volunteers after treatment with different regimens of recombinant human erythropoietin (r-HuEPO). Twenty-four subjects were randomly assigned to one of three different subcutaneous (SC) r-HuEPO (Protcrit; Ortho Biotech) administration protocols (I: 300 U/kg on days 1, 4, 7, 10; II: 400 U/kg on days 1, 5, 9; III: 600 U/kg on days 1, 10) with oral iron supplementation (Niferex; 150 mg, twice a day). The characteristics of the reticulocytes produced were examined with a flow cytometry method that allows measurements of individual reticulocyte cell volume, hemoglobin concentration, and hemoglobin content. Administration of SC r-HuEPO was associated with a significant increase in the production of reticulocytes. The hemoglobin content of reticulocytes (CHr, in picograms of hemoglobin per cell) in the three groups was 28.5+/-1.0, 28.2+/-0.5, and 28.5+/-1.3, respectively, at baseline, decreased to 24.6+/-1.6 (p< 0.001), 24.5+/-2.3 (p< 0.001), and 27.5+/-1.8 (not significant) at day 10, and returned to baseline after r-HuEPO was discontinued (28.8+/-0.9, 28+/-0.8, and 28.8+/-1.4, respectively, at day 22). The percentage of reticulocytes with cell hemoglobin content less than 23 pg was taken as an indicator of iron-deficient erythropoiesis. At baseline, 5.6%+/-2.7%, 6.9%+/-3.4%, and 8.3%+/-3.8% of reticulocytes had less than 23 pg hemoglobin in groups I, II, and III, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)