JAUNDICE exists when the bilirubin concentration in tissue fluids is higher than normal. The definition of the cause of jaundice, though difficult at times, is extremely important if proper therapy is to be instituted. The considerable information obtainable from laboratory tests, diagnostic procedures, and a careful medical history does not ensure the correct diagnosis. At the present time the increase in the number of cases of drug jaundice accentuates the problem of differential diagnosis. Of the drug jaundices, those with cholestasis are the most confusing to differentiate from mechanical obstructive jaundice. Although in the past cholestatic drug jaundice occurred infrequently today it predominates.1 Since the demonstration, by means of electron spin resonance spectrometry, that free radicals commonly occur in biological systems2 and are decisive intermediates in enzymatic oxidationreduction processes,3, 4 we have undertaken comparable investigations of surviving animal tissues. These studies show that most animal tissues