Mild solution conditions are described for the direct reassembly of a simple spherical virus, cowpea chlorotic mottle virus (CCMV), from its isolated protein and RNA components. Previously the reconstitution of spherical viruses has only been accomplished in conditions which are clearly non-physiological. The reassembly proceeds efficiently near neutral pH, at ionic strength 0·1 or 0·2 and 25°C and requires the protein to be in a non-equilibrium state. Addition of Mg2+ is not required. At pH 6, about 70% of the RNA is encapsidated as infectious virus. The 3 S protein aggregate (which assembles into the capsid) is monodisperse and consists of dimers of the subunit. The material is indistinguishable from native CCMV by physicochemical and structural characterization and also shows similar infectivity to native CCMV in a dilution assay on cowpea plants. However, the specificity of the protein for its homologous RNA is not strong and a number of plant virus RNAs and synthetic polynucleotides can be encapsidated with efficiencies comparable to that found for the CCMV RNA.