Using the IgG fraction of an antiserum against cord red blood cell (RBC) membranes (F-IgG), antigenic properties of RBC of newborns (n = 24) and patients suffering from anemia (n = 46) [either due to beta-thalassemia intermedia (n = 37) or hemorrhage (n = 9)] as compared to those of normal adults (n = 18) were examined with fluorescence microscopy, flow cytometry and radioimmunoassays (RIA). With fluorescence microscopy and flow cytometry 1.01 +/- 0.31 and 0.82 +/- 0.28% (mean +/- SD), respectively, of cord RBC and 0.79 +/- 0.31 and 0.53 +/- 0.28% of RBC from anemic patients reacted with F-IgG. RBC of normal adults showed virtually no F-IgG reactivity. In anemic patients there was a good correlation between the percent of F-IgG-reactive cells and the percent of reticulocytes, although the former were only two thirds of the latter; the ratio of F-IgG-reactive cells to reticulocytes was higher in posthemorrhagic anemia than in thalassemia. Moreover, double stainings revealed that the majority of F-IgG-reactive RBC were at the reticulocyte stage (80%), and coexpressed transferrin receptor (96%). Furthermore, the F-IgG-positive RBC correlated inversely with Hb levels. When RIA was employed, F-IgG binding to RBC of anemic patients and newborns was similar and considerably and significantly higher than that to RBC from healthy adults. The results demonstrate the reappearance in certain forms of anemia of F-IgG-reactive RBC, which are likely to represent a subpopulation of reticulocytes.