A method for plasma bilirubin determination is described. It is a modification of the Jendrassik and Grof alkaline diazo-coupling procedure and allows determination of both conjugated and unconjugated bilirubin in small amounts of plasma. Five commonly used diazo-coupling methods are compared with special reference to the influence of hemolysis in the sample. The Jendrassik and Grof method as modified by Nosslin and Michaëlsson is shown to have the smallest methodological error and to be insignificantly influenced by hemolysis in the sample, in contrast to the methods of Malloy and Evelyn, Lathe and Ruthven, Ducci and Watson, and Powell. The bilirubin standardization procedure, methodological errors, and the influence of hemolysis in the sample are discussed.