Abstract Sulfuric acid-ethanol fluorescence after short-term incubation provides an efficient and reliable determination of unconjugated plasma corticoids. Under the chosen conditions more than 95% of the fluorescence is due to cortisol or corticosterone. With slight modifications the method can also be used for erythrocytes. For urine the method is less specific but is convenient as a screening test. Normal values at various times of day were determined. In comparing these figures with those obtained by other methods, diurnal variation, time reference standards and the psychological situation of the subjects at the moment of the blood sampling, must be taken into account.