1, By adding table salt to the mesobilinogen solution containing IXαamalgan reduction, and making its osmotic pressure apporximately epual to that of the body fluid, when this solution is made to act both under aerobic and anaerobic conditions on the liver slice specimens obtained from normal rabbit, the concentration of mesobilirubinogen is reduced, especially its reduetion is marked under aerobic conditions. However, during this reaction bilirubin, mesobilirubin, porphyrin, and propentdyopent are not produced. Moreover, the concentration of IXα-urobilin after the reaction remains the same as before the reaction. 2. In the paper chromatograph and column chromatograph of the mesobilirubinogen solution containing IXα-urobilin as mentioned above and the solution after the reaction that were left standing at room temperature for 23 days, there can be detected the production of mesobiliviolin and mesobilirhodin. This fact suggests that the decomposition of mesobilirubinogen in the liver does not occur by oxidation alone. 3. Using the solution after the reaction mentioned in paragraph 1, column chromatography has been performed with active alumina as adsorbent agent and the layer containing mesobilifuscin is isolated; and then by making Schlesinger's reaction take place, lead acetate is prepared; and in the paper chromatograph of the last substance the existence of mesobilifuscin has been confirmed. 4. Although it is not puite certain but since the quantity of mesobilifuscin is much less than the quantity of mesobilirubinogen decreased in the reaction, there seems to be other decomposition products.