Three antisera specific to aldosterone and elicited with different aldosterone protein conjugates (aldosterone-3-oxine rabbit serum albumin and aldosterone-3-oxime bovine gamma-globulin) were studied by radioimmunological methods at various times subsequent to first-immunization. A considerable variability of the parameters important in radioimmunoassay was observed over the whole experimental period. Titer, sensitivity and specificity of two antisera tended to increase as long as the animals were boosted. In the third they did not change in a uniform way.