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The low plasma levels of corticotrophin (ACTH) and the variable non-specific effects of different plasma samples on the assay system make extraction and concentration desirable when ACTH is measured by radioimmunoassay. Currently available extraction procedures (Bornstein & Trewhella, 1950; Demura, West, Nugent, Nakagawa & Tyler, 1966) are time-consuming, and there is a place for a simple, rapid and reliable technique.

The affinity of ACTH for glass, talc, ion exchange resins, cellulose and charcoal was, in practice, not sufficiently reversible to be of use in the extraction of ACTH from plasma. It was found, however, that silicic acid (100-mesh Mallinckrodt) adsorbed over 90% of ¹³¹I-labelled porcine ACTH added to plasma, and yielded over 90% of the adsorbed tracer when eluted with a mixture of glacial acetic acid—acetone—distilled water in the proportion 1:25:100 (by vol.). ([¹³¹I]ACTH was prepared by J. Landon's modification of Greenwood, Hunter & Glover's 1963 technique.)