Fetal Hb (HbF)-bearing reticulocytes (F reticulocytes) could be detected in peripheral blood by a modification of the microscopic single-cell radial immunodiffusion method. Thereby otherwise inappreciable changes in HbF production could readily be recognized. F reticulocyte freqencies were reproducibly measurable whenever the product of whole blood HbF-bearing red cell (F cell) frequency and reticulocyte frequency was .apprx. 5 .times. 10-4 or greater. Serial analyses of F reticulocytes and nonreticulocyte F cells (F erythrocytes) illustrated the following: levels of F reticulocytes and F erythrocytes are persistently similar in normal adults with more than 6% F cells, and cell survival times of F and non-F cells must be essentially the same, changing levels of F reticulocytes can be sensitive predictors of later changes in mature F cell frequencies during infancy and in adults recovering from aplastic anemia and alterations in F reticulocyte frequency and the amount of HbF per F reticulocyte are discordant in some settings but concordant in others.