The precursor shells of dsDNA bacteriophages are assembled by the polymerization of competent states of coat and scaffolding subunits. The fluorescent dye 1,1′-bi(4-anilinonaphthalene-5-sulfonic acid) (bisANS) binds to both the coat and scaffolding proteins from the Salmonella typhimurium bacteriophage P22. It displays little affinity for the polymerized forms of the proteins. The subunits with bound bisANS are incapable of assembling into procapsids. The binding constants of bisANS for both coat and scaffolding protein monomers have been measured and are 7 and 6 µM, respectively. Binding of bisANS to coat protein has little effect on the conformation as determined by circular dichroism and susceptibility to proteolysis. Binding of bisANS to scaffolding protein induces a change in the secondary structure consistent with a loss of α-helix, and an altered susceptibility to proteolysis. We suggest that the bisANS is probably binding at sites responsible for intersubunit interactions and thereby inhibiting capsid assembly. © 1993, American Chemical Society. All rights reserved.