AbstractProgressive iodination of HSA affects its electrophoretic behaviour : however, no apparent formation of degradation products has been detected up to a substitution degree of about 24 iodine atoms per molecule of HSA.The HSA, progressively iodinated, shows an increased mobility on agar gel, and behaves as a single and homogeneous compound by moving boundary electrophoresis. No apparent correlation exists between the electrophoretic behaviour and the biological properties of the iodoalbumins which have been previously checked in humans. On this basis, the validity of electrophoresis as a control tool for the labelled albumin may be seriously questioned.