A method for colorimetric estimation of serum bilirubin by coupling with diazotized sulphanilic acid. The direct van den Bergh reaction, in aqueous medium, yields the 'direct' bilirubin, i.e. the amount of water-soluble bilirubin glucuronide conjugates. Upon prior addition of methanol to solubilize free bilirubin, the total bilirubin is found, and by this indirect van den Bergh reaction the 'indirect' bilirubin, i.e. the amount of unconjugated bilirubin, may be calculated as the difference between the total and direct levels.