A recently described reaction between barbituric acid and biliverdin has been investigated for suitability as a new approach to the colorimetric estimation of total bilirubin concentrations in hyman sera. The 6eaction was found to give absobance values which correlated well with bilirubin concentrations determined by an automated diazotisation method. As a manual technique, the sensitivity was greater than most methods based upon diazotisation. The reaction is likely to be of particular value when a method is required to estimate low levels of serum bilirubin.