A critical evaluation of existing colorimetric and spectrophotometric methods for the determination of bilirubin in blood plasma is presented. The stoichiometric equivalences of the reagents used in the diazo coupling reaction are re-evaluated. The elimination of turbidity is discussed, as well as methods using diazo-coupling promoters. Recommendations are made concerning the preservation of specimens to prevent oxidation and procedures that give maximal specificity for the type of bilirubin pigment being assayed.