We have newly synthesized a class of photochemical 1,8-naphthalimide dyes. Photochemical investigation strongly suggests that these dyes function as photoalkylation agents following activation to an intermediate state by visible light (circa 420 nm) excitation. The activated species reacts readily with nucleophilic amino acid residues, e.g., tryptophan, cysteine, and methionine. One dye, 1,14-bis-(N-hexyl-3'-bromo-1,8'-naphthalimid-4'-yl)-1,4,11,14- tetraazatetradecane-5,10-dione, which incorporates two reactive 1,8-naphthalimide groups at each end of an intervening structural bridge has been used to cross-link the protein monomers of F-actin, thus preventing its natural depolymerization at low salt concentrations, and to cross-link Apolipoprotein I of human high-density lipoprotein. These observations suggest continued study of these dyes as agents for protein cross-linking, tissue bonding, and inactivation of infectious agents.