Abstract A highly sensitive and specific antiserum was obtained by immunizing a rabbit with an aldosterone-carboxymethoxime bovine gamma globulin conjugate. Immunological analysis of the antiserum was performed 6, 9, 13 and 23 weeks after first immunization. Over the observed period the antibody titre ranged from 4000 to 40,000 and antibody concentration was between 410 and 3540 μg/ml antiserum. Association constants were in the order of 10 9 1./mole. When antiserum from week 23 was used, 50% displacement of [ 3H]-aldosterone bound to the antibody was achieved by the addition of 15 pg unlabeled aldosterone, indicating very high sensitivity. The same effect was seen with sera from week 6, 9 and 13 when 32, 23 and 21 pg of unlabeled aldosterone were added. The minimal amount of hormone which produced a statistically significant decrease in [ 3H]-aldosterone bound to the antibody was 3 pg with antiserum from week 6, whereas sera from week 9, 13 and 23 allowed the detection of 2 pg alsosterone. Over the experimental period antisera showed cross reaction with cortisol ranging from 0.0007 to 0.0010% and with corticosterone from 0.12 to 0.06%. The high sensitivity and high specificity of the antiserum allowed the determination of plasma aldosterone in the presence of other naturally occurring steroid hormones.