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The significance of the various types of direct van den Bergh reaction is discussed, and it is emphasized that the so-called biphasic reaction is not due to the presence in the serum of two forms of bilirubin, but to the failure of strong solutions of azobilirubin to follow Beer's Law. The terms direct and indirect bilirubin should be abandoned because there is no proof that these two forms really exist. The use of the term direct-indirect quotient (D.I.Q.) is recommended since it merely gives an expression of the extent of the direct reaction under the conditions of the experiment. Neither the direct van den Bergh reaction nor any of its modern modifications enable one to distinguish between the jaundice of acute hepatitis and that due to obstruction. For clinical purposes, the method of Malloy and Evelyn is the simplest and best of those examined. A D.I.Q. below 40 is then diagnostic of retention jaundice, and a D.I.Q. above 50, although usually indicative of regurgitation jaundice, can occur in retention jaundice complicated by liver damage. Grants from the Central Research Fund of the University of London, and from the Medical Research Council, enabled special necessary apparatus and chemicals to be purchased.