1. Lactoperoxidase was extracted from cow milk by a simplified method starting with batch-wise adsorption onto GM-Sephadex-50. It was then purified by (NH4) 2SO4 precipitation and isoelectric focusing. This product had an A412 nm/A280 nm ratio of 0.8-0.9. 2. Lactoperoxidase together with H2O2 could oxidize carrier-free Na125I to" active iodine" with efficiency to iodinate proteins to high specific activity. 3. Polypeptide hormones radioiodinated by this technique retained their immunological reactivity and were used in radioimmunoassays with good results.