A radioimmunoassay for the measurement of 18-OH-DOC in plasma was developed using an antiserum raised against the gamma-lactone derivative. The steroids with the greatest degree of cross reaction were 18-OH-corticosterone-gamma-lactone and aldosterone-gamma-lactone which showed cross-reactivities of 1.96% and 0.47% respectively. These and other interfering steroids were eliminated by chromatography of the extracts on columns of Sephadex LH-20. The lowest limit of detection of 18-OH-DOC in 1 ml of plasma corresponded to 33 pmol-1. The intra-assay precision was 9.7, 4.8 and 2.6% at 102.0, 316.1 and 1144.0 pmol 1(-1) respectively and the interassay precision was 15.3 and 5.4% at 71.3 and 404.7 pmol 1(-1) respectively. The amount of 18-OH-DOC measured (y) which showed a high degree of correlation (r = 0.999) with the amount added (x) to plasma in the range 240--1920 pmol 1(-1) could be predicted from the linear least squares equation y = 1.006x + 31.3. The concentration of 18-OH-DOC in ten normal subjects was 172.1 +/- 39.1 pmol 1(-1) at 09.00 h, 100.9 +/- 16.9 pmol 1(-1) at 12.00 h and 95.8 +/- 33.3 pmol 1(-1) at 16.30 h. Plasma 18-OH-DOC and cortisol levels were measured after various intravenous doses of ACTH in three patient with esential hypertension. Lower doses of ACTH caused similar percentage increases in both hormones but higher doses caused considerably greater increases in 18-OH-DOC. These results confirm the ACTH dependancy of 18-OH-DOC secretion.