Precise determination of reticulocytes (young red blood cells) containing fetal hemoglobin (Hb F), F-reticulocytes, is important for assessment of the efficacy of drugs such as hydroxyurea and butyrate in elevating the levels of Hb F in patients with sickle cell disease (SCD) and beta-thalassemia. We developed a reliable and easily applicable method for determining F-reticulocytes using fluorescence image cytometry. Reticulocytes were first identified by preparing a monolayer smear of blood stained by acridine orange. Images of reticulocytes and of all cells were obtained for a selected area on the smear. After removing the acridine orange, cells containing Hb F (F-cells) in the same area were then identified by immunofluorescence. Using images of F-cells, reticulocytes, and all cells for the same fields it was possible to identify F-reticulocytes. To assess the validity of our two-stage staining method, we compared our results with those obtained by traditional methods. There was significant correlation of our method with the conventional immunofluorescence staining method for F-cells (r2 = 0.99; slope = 0.99) and with the accepted brilliant cresyl blue method for reticulocytes (r2 = 0.97; slope = 0.96). Heretofore, the ability to determine F-reticulocyte levels has been limited to a small number of laboratories possessing special equipment and techniques. The method presented here should be of great interest to many basic science and clinical investigators involved in studies evaluating synthesis of Hb F.