Serum albumin-gold complexes exhibit UV-excitable red luminescence (λem = 640 nm) with unusual Stokes shifts compared with the innate UV/blue fluorescence arising from the aromatic residues. In order to understand the mechanism of this luminescence, we employed limited proteolysis and molecular cloning techniques and assessed the domain containing the red luminophore in bovine serum albumin (BSA) and human serum albumin (HSA). We identified that the luminophore is localized in a domain of serum albumin, residing within the N-terminus half.