The delivery of transferrin iron from four animal species and man to homologous reticulocytes was measured at different transferrin saturations. Total iron uptake in the in vitro reticulocyte incubation model employed followed a hyperbolic curve, increasing as the transferrin saturation increased but at a progressively slower rate. In all species, there was a much greater iron delivery from diferric as compared to monoferric transferrin, the molecular advantage varying from 8:1 to 14:1. The majority of iron was delivered from diferric transferrin when transferrin saturations exceeded 13-19% depending on the species. Thus a general similarity exists in the transferrin-iron receptor interactions in these mammalian species. Formuli have been provided whereby the iron utilization curve may be calculated when uptake has been determined at any one transferrin saturation.