The fluorescence of DNSA (5-dimethyl-aminonaphthalene-1-sulfonamide) in solution with human serum albumin has been used to study the binding characteristics of DNSA to this protein. Such binding is apparently hydrophobic in nature and occurs near the tryptophan residue of the protein. Various drugs displaced DNSA from human albumin in direct proportion to their own affinities for plasma protein. An association constant of 3.68 × 104 1/m was approximated for the binding of DNSA to human albumin.