Synthetic LH—RH was iodinated with 125I by the lactoperoxidase technique. The product was separated from non—iodinated LH—RH by polyacrylamide gel electrophoresis (PAGE), identified as the monoiodo—derivative, and shown to retain biologic activity. Iodinated oligomers of LH—RH were separated by PAGE.(Endocrinology92: 1725, 1973)