Competitive binding of bilirubin and 5-butyl-1-cyclohexylbarbituric acid (bucolome) to human serum albumin was investigated by equilibrium dialysis, with and without added bilirubin (0.5 mol/mol albumin). Further observations were made by peroxidase oxidation kinetics. Finally, cobinding of the two ligands was studied by spectrophotometry. Experiments were performed with defatted adult human serum albumin, with umbilical cord serum, and with adult albumin to which was bound 2 mol palmitate/mol albumin. Bucolome does not induce displacement of bilirubin from binding to defatted adult albumin when one molecule of either ligand is bound. The second molecule of bound bucolome displaces the first bilirubin molecule. With albumin in umbilical cord serum, the first bucolome does decrease binding of the first bilirubin to some extent although the two ligands can bind to the same albumin molecule. The presence of two molecules of bound palmitate together with one of bilirubin causes enhanced bilirubin displacement on binding of bucolome. These observations may be of consequence for practical testing of the bilirubin displacing effect of drugs. Experiments with neonatal albumin and with added fatty acid should be included for a complete study.