A direct radioimmunoassay for aldosterone in unextracted plasma was developed using a highly sensitive and specific antiserum and 125I-labeled aldosterone. This new method was compared with the previously used assay based on methylene chloride extraction and column chromatography and with a micromethod utilizing methylene chloride extraction alone. Concentrations of 2 pg/ml aldosterone can be measured in 25 microliter plasma. The within-assay coefficient of variation ranged between 4.0% and 5.3% over a sixfold range of aldosterone concentrations, and the between-assay coefficient of variation ranged between 5.0% and 9.4% over a tenfold range. The correlation coefficients among the three methods varied between 0.97 and 0.99, and results were virtually identical between the new direct and the standard chromatography methods. Normal values on unrestricted salt intake were 68 +/- 28 pg/ml (n = 30, mean +/- SD) in seated humans. In conclusion, this is an extremely simple, truly direct microradioimmunoassay for plasma aldosterone that is accurate over a wide range of plasma concentrations, is exquisitely sensitive, and provides results identical to those obtained with established methods.