The synthesis and testing of a new type of nucleic acid-protein photocrosslinking reagent is described. The reagents are composed of a psoralen ligand for nucleic acid photoattachment, which is linked to an azidobenzoyl group, for protein photoattachment. The linker contains a disulfide bridge which can be opened by reduction with mercaptans. The reagents were tested in a chromatin system, where it was found that they induced cleavable crosslinks between the histones and the DNA upon irradiation with long-wavelength ultraviolet light (lambda greater than 300 nm).