Erythroid cells in the marrow express CD71, transferrin receptor, and reticulocytes released from the marrow lose their expression during maturation. The immature reticulocyte fraction, the proportion of reticulocytes with the highest content of RNA, has been determined by hematology analysis. In the present study, we examined CD71 expression on immature reticulocytes by flow cytometry (FCM) in paroxysmal nocturnal hemoglobinuria (PNH) patients with reticulocytosis. We modified 'reticulocyte-gated FCM' to multi-color FCM, i.e. RNA/CD71, RNA-CD59 or CD59/CD71/CD45. In PNH, in addition to the increased number of immature reticulocytes (%CD71-positive), a more immature phenotype in regard to both CD71 intensity and RNA content levels was demonstrated. In seven PNH patients studied, %CD71-positive reticulocytes were significantly increased at 32.2 +/- 11.9% (n = 10, normal 10.4 +/- 3.5%, P = 0.002). RNA content levels (assessed by mean fluorescence index, MFI) in CD71-positive reticulocytes were significantly increased at 812.0 +/- 215.2 MFI in PNH (n = 10, normal 508 +/- 86.1 MFI, P = 0.002). These data indicate that stimulated erythropoietic conditions induce the release of more immature reticulocytes to the peripheral blood than ordinary erythropoietic conditions. CD71 intensity on immature reticulocytes was well correlated with their RNA content levels, indicating the usefulness of CD71 as an immature reticulocyte marker.