The intrinsic fluorescence of proteins arises principally from the aromatic amino acids tyrosine and tryptophan. Fluorescence of tryptophan predominates even in proteins containing much more tyrosine than tryptophan, for a variety of reasons including the relatively high absorbance of tryptophanyl residues, low fluorescence quantum yields of most tyrosyl residues and the existence of efficient mechanisms for the transfer of excitation energy from tyrosyl to tryptophanyl residues [l]. Even HSA, which contains 1 tryptophanyl and 18 tyrosyl residues, displays fluorescence characteristic of tryptophan and a sophisticated mathematical analysis was required to demonstrate the tyrosyl contribution [2].