1. A procedure is described for preparing 131I-labelled adrenocorticotrophin suitable for use in radioimmunoassay. 2. Adsorption of labelled and unlabelled adrenocorticotrophin at low concentrations occurs to various surfaces despite the presence of diluent protein. Adsorption and desorption errors are minimized by low pH and by the use of polystyrene vials. 3. Preparations with low initial damage are obtained if the radioiodination is performed rapidly and the separation of 131I-labelled adrenocorticotrophin from unchanged [131I]iodide is carried out on cellulose columns by using dilute acid. 4. The immunological activity of 131I-labelled α1–24-adrenocorticotrophin, but not of 131I-labelled porcine adrenocorticotrophin, decreases with increasing specific radioactivity. The involvement of tyrosine residues in the immunological specificity of the α1–24-adrenocorticotrophin only is suggested to explain this finding.

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