1. According to the Fletcher-Huehns hypothesis there exists a functional difference between the two iron-binding sites of transferrin. 2. The aim of the study presented was to evaluate this hypothesis in a homogeneous system, with human bone marrow cells and pure human monoferric transferrins A and B. 3. For this reason normal human bone marrow cells were incubated with human monoferric transferrin. The monoferric transferrins A and B were obtained by selective labelling at different pH of apotransferrin followed by preparative isoelectric focusing in granulated gels. The uptake of iron by the cell suspensions from monoferric transferrins A and B was equal. 4. In a heterogeneous but more active system for the removal of iron from human transferrin in vitro the two human monoferric transferrins did not show any significant functional differences. 5. No support for the Fletcher-Huehns hypothesis could be obtained.