We previously described an RIA for aldosterone based on highly specific antibodies elicited with aldosterone-3,20 dioxime bovine γ-globulin (Poulsen, K., J. Sancho, and E. Haber, Clin Immunol Immunopathol2: 373, 1974). We now report the development of higher affinity antibodies of similar specificity that allow the direct measurement of normal aldosterone concentrations in extracts of 100 μl plasma. The detection limit at ± 2 SD of mean zero value was 0.2–0.3 pg. The assay was validated by 1) zero values in adrenalectomized plasma, 2) parallel displacement plots of aldosterone standards and plasma extracts, 3) 97.7 ± 2.5% recovery of added tracer, 4) a correlation coefficient of 0.992 and a slope of 1.08 in a plot of added vs. recovered unlabeled aldosterone, 5) 10.2% interassay variation and 5.6% intraassay variation, and 6) failure of high concentrations of a variety of steroids added to plasma to perturb the measured aldosterone concentration. The simplicity of the assay results in a high degree of productivity, in that 195 plasma samples may be processed in duplicate, in addition to standard curves, in a single working day.