P-Hydroxyphenylpyruvic acid, not a product of intermediary metabolism but well tolerated, and catabolized by the liver, is a suitable substance for a liver function test. When the intensity of Millon''s reaction, detd. photometrically on 24-hr. urines, is expressed in mg. p-hydroxyphenylpyruvic acid excreted/day, the range for normal persons is 100-1000 mg. of the acid/day, but fairly uniform for each individual. After a 2 g. dose of the acid, normal subjects promptly oxidized it and excreted less than 10% of the dose in the 24-hr. urine; 2 patients with liver obstruction and myocardial damage excreted 20-30%, patients with cirrhosis of the liver 30-90%, a patient with carcinoma of the liver 38%, and patients with infected wounds 30-60%. Toxic substances resulting from infection may have an injurious effect on the liver desmoenzyme concerned in the oxidation; the excretion was often increased in infectious hepatitis, because of injury to the liver parenchyma cells. In 2 patients with thyreo-toxicosis, the acid was poorly oxidized, a patient with hemolytic jaundice oxidized the acid normally. Treatment with arsenicals frequently increased the excretion. The excretion of p-hydroxyphenylpyruvic acid and urobilinogen usually paralleled each other. This new test is more sensitive than other liver function tests.