A method for inducing highly specific cortisol antibodies for measuring plasmatic cortisol by RIA is described. The method is fast, direct (without extraction or separation), with high specificity, due to the high quality of the antibody produced in our laboratory. Reliability was demonstrated by: the sensitivity of the method, with a pattern curve of 50 pg of cortisol; the precision tests showed a variation rate intranalysis of < 9.7% and internalysis of < 17.6%; accuracy was > 87%; and specificity was demonstrated by antiserum characterization with other steroids. Measurings of cortisol in plasma of normal subjects could detect the circadian cycle pattern as well as normal supression with dexametasone. These findings support that our method allows fast measurement of plasmatic cortisol with accuracy and sensitivity, reducing operative cost.