A subset of sickle cells have an increased density at the reticulocyte stage of development, indicating that they are either abnormally dense upon release from the bone marrow or become dense quickly in the circulation. These cells are of interest because they most likely have severely disrupted cation regulation and a short lifespan. Based on the distribution of fetal hemoglobin (HbF) in the density fractions of sickle red blood cells (RBCs) and in vitro studies of cellular K+ loss, it seems likely that HbF content is an important in vivo determinant of dense cell formation. In this study, we tested the hypothesis that young, dense cells have low HbF content. Sickle RBCs were first separated into light and dense fractions. Reticulocytes were isolated from unfractionated cells and from each density fraction with an immunomagnetic technique directed against transferrin receptors (TfR) and assayed for the percentage of HbF and K+/Hb ratio. TfR+ reticulocytes isolated from unfractionated cells had a much lower HbF content when compared with all the unfractionated RBCs. This is most likely caused by enrichment of F cells because of a longer circulation life span. Heavy TfR+ reticulocytes had a K+/Hb ratio similar to that measured in the entire dense population and contained very low levels of HbF, averaging 2.5% of the level in all RBCs, 11.7% of the level in all TfR+ reticulocytes, and 4.0% of the level in all dense RBCs. These findings suggest that TfR+ dense cells derive predominantly from non-F cells. Furthermore, the amount of HbF in the circulating dense cells suggests that many of these cells do not derive from the TfR+ dense cells.