We have studied the interaction, in vitro, between diferric transferrin (FeTr), aluminum transferrin (AlTr), and human reticulocytes harvested from human placental blood. In particular, we aimed to determine the extent to which the kinetics of AlTr and FeTr differed. Using transferrin labeled with either 59Fe or 125I, the association of radiotracer with reticulocytes, as a function both of time and of transferrin concentration, was examined. Under the conditions of the experiments, the data are consistent with a mechanism involving at least three processes. Two early processes acting in parallel behave as a high-affinity saturable receptor and a low-affinity non-saturable receptor, neither of which distinguish between AlTr and FeTr. In a subsequent process, AlTr and FeTr exhibit different kinetics. This third process may be saturated by FeTr but not by AlTr. Interpreted in terms of a current conventional view of metallo-transferrin uptake, we conjecture that the early parallel processes involve cell surface phenomena including classical transferrin-receptor binding, and that the subsequent process represents events, possibly intracellular, involved in metallo-transferrin dissociation or further iron transport. The extent to which AlTr influences the interaction of FeTr with reticulocytes offers insight into both the normal physiology of iron uptake and the potential for toxicity by aluminum.