The bilirubin-diazo reactions have been investigated by using thirty six kinds of diazotized aromatic primary amines. Twenty nine of these diazo compounds yield red-violet coloured products with absorption maximum at about 500-550 nm, and the others green coloured products with absorption maximum at about 700nm. The latters are the products from the diazonium salts of o-, p-anisidine, o-, p-aminophenol, 2, 4-dinitroaniline, 2, 6-dichlo ro-4-nitroaniline, and p-aminoacetanilide.
It was assumed that the green coloured products are not the azobilirubin but the biliverdin based on the study of the spectrophotometric, kinetic properties of the reaction. The oxidation of bilirubin to biliverdin may be due to nitrous acid remained unreacted in the diazotization of aromatic primary amines.