Summary The properties of a mutant of cowpea chlorotic mottle virus which, although infective, cannot be disassembled in 1 m-NaCl, pH 7.5, are described. The mutant coat protein contains a lysyl to arginyl replacement which affects the hydrodynamic and titration characteristics of the virus which are discussed in regard to the anomalous ionization of a carboxyl-carboxylate pair and a lysyl residue.