A new mixed-complex of La(C7H5O5S)(phen)(3+) was synthesized. Resonance light-scattering characteristics of the interaction between La(C7H5O5S)(phen)(3+) with bovine serum albumin (BSA) were studied. After BSA was added, aggregation of La(C7H5O5S)(phen)(3+) on the molecular surface of the bovine serum albumin occurred at the pH 4.5 similar to 6.2, which resulted in an enhanced resonance light-scattering (RLS) peak at 400 nm. The intensity of resonance light-scattering was found to be proportional to the concentration of BSA.