The influence of pH changes upon the iron-binding properties of transferrin was investigated in the absence of chelating agents. The effects were demonstrated by spectrophotometry, gel filtration, and by studies of the intermolecular transfer of 59Fe from transferrin to conalbumin. At pH values below 6.7, diferric transferrin readily loses iron. The monoferric molecule, which is relatively resistant to acid dissociation, is preferentially formed. A temporary reduction of pH provides a simple method for selectively attaching iron to one metal-binding site, and allows double isotopic labelling of the transferrin molecule. This technique may permit further investigation of the physiological properties of the two iron-binding sites.