Using the solvent-partition method of Eberlein, 264 icteric serum specimens from 58adult patients with various hepatobiliary diseases (acute viral hepatitis 28, chronic hepatitis2, liver cirrhosis 7 cases, etc.) were analyzed serially for free bilirudin and conjugatedbilirubin (pigments I and II). There was a good correlation between the total and conjugated bilirubin concentrationsas determined by the solvent-partition method, and the total and direct reactingbilirubin concentrations as measured by the standard diazo reaction, respectively. Free bilirubin and total serum bilirubin tended to rise and fall in parallel. In casesof heptocellular jaundice, obstructive jaundice, Dubin-Johnson syndrome and Rotor syndrome, free bilirubin accounted for about 10 to 35 per cent. of total serum bilirubin, but a higher fraction (over 35 per cent.) occured when the total serum bilirubin con-.centration was less than 2 mg per dl. In patients with hemolytic jaundice or non-hemolyticacholuric jaundice, the percentage of free bilirubin exceeded 65 At any total serum bilirubin level. As described by Schachter the estimation of the fraction of total conjugated bilirubinpresent as pigment I (I/I+II) was carried out. This fraction is designated as M. During the course of acute viral hepatitis, Weil's disease and obstructive jaundice, the M value ranged between 0.31 and 0.63 in the increasing stage of jaundice, exceedingthe values in the peak and decreasing stage. In chronic hepatitis M was greater (0.30-0.41) in the active stage with icterus than in the latent, non-icteric stage. In casesof liver cirrhosis, the value was larger (0.31-0.65) in the period of obvious hepatic failurethan in the period when the hepatic dysfunction was well compensated. The M value and total serum bilirubin concentration altered almost in parallel, butthe change of the former as SGO-T activity alteration preceded that of the latter. Thus, although the estimation of M value offers little help for the differential diagnosisof jaundice or indication of the severity of liver cell destruction, the serial determinationin individual case can be an aid for forecasting the trend of jaundice andtransition of the phase of illness to a certain extent.