A method for the production of the haptens 18-hydroxy-11-deoxycorticosterone-3-(O-carboxymethyl)-oxime (18-OH-DOC-3-CMO) and 18-hydroxycorticosterone-3-(O-carboxymethyl)-oxime (18-OH-B-3-CMO) is described. The formation of the oximes was studied in kinetic experiments. They were prepared at pH 1.6 in methanol/HCl using a short reaction time. Antisera were raised in rabbits using serum albumin conjugates. The highly specific antisera were used at a final dilution of 1:79 000 (18-OH-DOC) and 1:43 000 (18-OH-B); the affinity constants were 1.2 x 10(10) l/mol and 8.1 x 10(9) l/mol, respectively. The radioimmunoassay procedure for 18-OH-B in serum involves purification by paper chromatography. The intra- and interassay precision was 7.3% and 12.3%, respectively. The mean serum 18-OH-B level (+/- S.D.) for normal male and female ambulatory subjects (n = 40) on a normal sodium diet was 0.802 +/- 0.262 nmol/l. After 60 minutes of recumbency, the serum 18-OH-B level was 0.313 +/- 0.061 nmol/l (n = 6) for men.