A method for measuring aldosterone concentration has been improved by extracting the steroid from plasma with an antibody prior to radioimmunoassay. Precision, sensitivity and specificity were comparable in our laboratory to a method using paper chormatography and to published data for other methods. Duplicate specimens assayed with both methods gave a correlation of r = 0.91 (p < 0.005). This method eliminates chromatography and enables one technician to process 80–120 specimens each week.