Seromucoid was determined in 165 patients with jaundice or hepato. megaly, or both. Subnormal seromucoid concentrations were found in 39 per cent of 44 cases of viral hepatitis and in 15 per cent of 41 cases of alcoholic cirrhosis. This incidence increased to 50 and 43 per cent, respectively, when patients with complicating extrahepatic disease processes were excluded. No subnormal values were encountered in 55 cases of benign and malignant biliary obstruction which, however, yielded about 80 per cent of elevated values. The empirical data point to the lower limit of the normal seromucoid range as providing a useful dividing line between obstructive and parenchymal jaundice: the finding of a subnormal level is strongly indicative of the latter. Normal or elevated levels may only occasionally be expected to make a decisive diagnostic contribution. A more definitive evaluation of the clinical utility of this type of test must await improvements in analytic standardization, based on a more detailed knowledge of the chemical and physiologic identity of the compound than is available at present.