Insulin was labelled with /sup 125/I using lactoperoxidase as an oxidizing agent. The reaction product was purified via two stages; a starch gel electrophoresis(SGE) and a Sephadex gel filtration(SF). Upon comparison of the labelling yields and the bindabilities of the labelled insulin to its antibody, it has been found that the enzyme method shows higher yields (50%) and the better bindability to its antibody than the conventional chloramine-T method (35%). By checking the insulin blank labelling mixture with a SGE, a paper chromatography, and a radioautography technique, a by-product in the lactoperoxidase method has been identified. The separated fractions in SGE and SF were also analyzed and discussed.