A radioimmunoassay for 21-deoxycortisol is described. The immunogen, 21-deoxycortisol-3-(0-carboxymethyl) oxime-bovine serum albumin, was prepared, the antisera raised against it were studied and the reliability of the assay was checked. The antiserum selected cross-reacted with 11-deoxycortisol (0.08%), corticosterone (0.25%), cortisol (0.6%) and 17-hydroxyprogesterone (1.6%). 21-deoxycortisol was separated by celite partition chromatography and eluted in the 70/30 (v/v) isooctane/ethyl acetate fraction together with 11-deoxycortisol and corticosterone. The radioimmunoassay was used to measure 21-deoxycortisol in the plasma of normal subjects and patients with androgen excess. In normal subjects, men (0.19 ng/ml +/- 0.08) and women (0.18 ng/ml +/- 0.09) had similar basal levels (mean +/- SD). One hour after ACTH stimulation, these levels were increased by a factor of 3.5. In 7 patients treated for classical congenital adrenal hyperplasia associated with 21-hydroxylase deficiency, basal values varied between 9.1 and 39.9 ng/ml (measured at 8 a.m.). In 7 untreated women with late-onset congenital adrenal hyperplasia (with 21-hydroxylase deficiency), ACTH-stimulated levels were increased to between 9 and 25.5 ng/ml. In 14 heterozygous carriers of 21-hydroxylase deficiency, diagnosed by HLA genotyping, all ACTH-stimulated levels were well above the highest corresponding levels in normal subjects, whereas 17-hydroxyprogesterone levels remained within the normal range in 9 of the cases.