The reduction of biliverdin was attempted by dissolving biliverdin in various media with the cultivation of either E. coli communis and the filtered bacterium solution. And the results were as follows. 1. In the non-proteinous media or in the pepton aqueous solution as the medium provided in the presence of 0.1% glucose biliverdin is reduced to bilirubin. However, in this instance the reduction takes place only at the central methin group but not at the vinyl group of side chain. 2. Even in the presence of pentose or hexose besides glucose similar result can be obtained, though the reduction power is less. In this instance sugar alone can not produce bilirubin. 3. Even under the condition unfavorable to the growth of bacterium, in the presence of glucose a small quantity of bilirubin is produced. 4. When various amino-acid sulfur compounds, l-ascorbic acid or egg-white exudate solution are added to the medium along with glucose, such addition only affected the velocity of the reduction and gave the same result. 5. In the presence of these substances added to various media a pigment possessing the characteristics of direct birect bilirubin has been isolated simultaneously. 6. Under the aerobic condition no effect other than the slowing-down of the reduction velocity can be recognized. 7. The factor that plays a role in the reduction of biliverdin is also excreted outside the body of E. coli communis during the culture. Furthermore, this factor is heatresistant and its reduction capacity shows itself only in the presence of glucose.