A direct, simple and highly specific radioimmunoassay for corticosterone has been developed. The assay does not require preliminary solvent extraction of the sample or any chromatographic step. The assay utilises a highly specific antibody raised in rabbits against corticosterone-3-(0-carboxymethyl)-oxime-BSA immunogen and gamma-labeled corticosterone of high specific activity. An excellent correlation was obtained between results of the direct assay and those measured after paper chromatography (r = 0.99, P less than 0.001). The coefficients of variation for intra-assay and inter-assay determinations of samples from normal and high plasma pools were 4.6-6.2% and 6.4-8.2% respectively. The minimum limit of detection was 5 pg/assay tube (0.1 ng/ml). The assay has been applied to assess plasma corticosterone levels in various physiopathological and pathophysiological studies. It is extremely practical to the extent that a single technician can assay up to 1000 samples in a working week. Finally, the direct assay has been validated and employed for in vitro adrenal superfusion studies using either rat or human adrenal cells. The large numbers of samples produced by these studies would have exceeded the capacity of earlier radioimmunoassays requiring initial extraction and chromatography.