The identification of "direct reacting" bilirubin in bile coincided with the classical studies of G. J. Dutton and I. D. E. Storey, in which they showed that various alcohols and acids could be converted to glucuronide conjugates by a microsomal enzyme with uridine diphosphate glucuronic acid (UDPGA), as the glucuronyl donor. The conditions for optimum conjugation differed slightly from those for o-aminophenol, and it was postulated that at least two uridine-5’-diphosphate (UDP)-glucuronyltransferases existed and a deficiency of bilirubin glucuronyltransferase was responsible for jaundice in the newborn rather than a limited production of UDPGA. The initial methods employed for the determination of bilirubin glucuronyltransferase activity were dependent on the "direct" Van de Bergh reaction. UDPGA is more readily available than bilirubin, which has to be prepared biosynthetically. In vitro, most assays are performed in the presence of detergents, which are known to disturb the kinetic properties of the enzyme, and with high concentrations of bilirubin.