Relative equilibrium constants ("affinity ratios") of complexes of bilirubin with a molar excess of charcoal-treated serum albumins from different species (human, bovine, rabbit and chicken) in aqueous solution, were estimated by circular dichroism measurements in the visible region at 26-27 degrees C, pH 7.4, and in the presence of 0.1 M NaCl. By variation of the mol ratios of the components of pairs of different bilirubin-serum albumin complexes showing circular dichroic bands of opposite sign, the apparent association constants of complexes of bilirubin with either human or chicken albumin were found to be greater by factors between 6 and 17 than those of bovine or rabbit albumins. The usefulness in the determination of affinity ratios is illustrated by the evaluation of single equilibrium constants of systems of high-ligand affinity from those of relatively lower affinity, the latter of which are more readily amendable to direct experimental measurement.