The preparation and immunological properties of I-labelled adrenocorticotrophin.

1. A procedure is described for preparing (131)I-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 (131)I-labelled adrenocorticotrophin from unchanged [(131)I]iodide is carried out on cellulose columns by using dilute acid. 4. The immunological activity of (131)I-labelled alpha(1-24)-adrenocorticotrophin, but not of (131)I-labelled porcine adrenocorticotrophin, decreases with increasing specific radioactivity. The involvement of tyrosine residues in the immunological specificity of the alpha(1-24)-adrenocorticotrophin only is suggested to explain this finding.