A competitive dialysis technique has been used to study the relative affinities of the two iron-binding sites on transferrin molecules and the relative binding strengths of transferrins isolated from plasma of different species. 2. The comparisons were extended to include desialylated human transferrin, ovotransferrin, and a cyanogen bromide fragment of the latter. 3. Although the results of bilateral experiments could generally be accounted for in terms of the theory of independent sites, there were some exceptions, and cyclic comparisons were inconsistent. 4. All the comparisons made were compatible with a model in which site-interaction occurred, but it was not possible to decide whether the sites were intrinsically identical or not. For most species this corresponded to positive cooperativity, but for rabbit it was negative. 5. The average affinity of transferrin for iron depended on species, but the variation was never more than about one order of magnitude. 6. No effect on the binding constants for human transferrin could be detected when the sialic acid residues were removed. 7. The fragment of ovotransferrin competed fairly effectively with the native molecule for iron, although the average relative affinity was only about 1:15. 8. The relative binding of iron by ovotranferrin and human transferrin was affected little when bicarbonate anion was replaced by oxalate, although the ratio of the two binding constants for ovotranferrin increased.
