Nonenveloped viruses provide the chemist with large, preassembled polyvalent protein scaffolds for modification. These structures are typically porous to small molecules but not to large ones. The solution-phase structures and reactivities of such assemblies may be substantially different than indicated by X-ray crystal structures. Here, the attachment of organic compounds to either the inside or outside surface of the cowpea mosaic virus (CPMV) coat protein was verified with an indicating antibody-antigen interaction. Antibody binding was subsequently blocked by the installation of poly(ethylene glycol) chains. These results typify the type of site-specific control that is available with CPMV and related virus building blocks.