The method of estimating bilirubin in the blood that was introduced by Hijmans van den Bergh is extensively used in clinical medicine. It differs from other methods of estimating bilirubin in blood serum or plasma since it not only allows quantitative estimation of bilirubin, but reveals qualitative differences by which jaundice of hemolytic and obstructive origin can be differentiated. Van den Bergh rediscovered the color reaction given by bilirubin with the diazo reagent of Ehrlich, and applied this reaction in producing an accurate test for small amounts of bilirubin in small amounts of serum. With regard to the delicacy of the test, he found that a solution of 1: 1,500,000 of pure bilirubin dissolved in alcohol still affords a positive result. Next he made the original and significant observation that in some cases of jaundice it is unnecessary to bring the bilirubin into alcoholic solution, but that a positive coupling