Comparison between human 125I-labelled TSH labelled with lactoperoxidase or chloramine-T; advantages in the use of enzymically labelled antigen in a radioimmunoassay for human TSH.

1. Human thyroid stimulating hormone (human TSH) was iodinated by both chloramine-T and lactoperoxidase methods. 2. Human 125I-labelled TSH prepared by the enzymic method showed less damage on storage, an increased shelf-life, and improved binding at different antiserum dilutions. 3. A comparison of the values of the bound counts on the addition of human TSH, expressed as a percentage of the zero standard bound counts (B/BO values) in the standard curves, demonstrated that lactoperoxidase-labelled human TSH gave increased sensitivity.