4Z,15Z-bilirubin-IXa(4Z,15Z-BR), an endogenous compound that is spar-ingly soluble in water, binds human serum albumin (HSA) with high affin-ity in a flexible manner. A phage library displaying recombinant HSA domain II was constructed, after three rounds of panning against immobi-lized 4Z,15Z-BR