Isotopically labelled protein or polypeptide hormones are chemically and/or structurally different from the native material. This may be a major handicap when biological studies or radioimmunoassays are performed. Labelling procedures with 131I, 125I or by introduction of 3H (exchange, acetylation, alanylation) are described, and the advantages and disadvantages of the different methods are outlined. The specific activity and biological and immunological properties of the labelled end products are discussed. The review presented should enable the individual investigator to choose the appropriate method of labelling according to his specific requirements.