The inner capsid structure of the OSU strain of porcine rotavirus was studied by electron microscopy of freeze-dried preparations and of negatively stained chemically disrupted virus particles. The analysis of the particles by the freeze-drying technique revealed a T:13 l (laevo) symmetry for the organization of the inner capsid. Treatment of single-capsid rotavirus particles with 30% formamide or 5 M-urea resulted in their degradation, giving rise to very similar products, corresponding to isolated vertices, edges and faces of the virus icosahedron. An analysis of such structures confirmed the triangulation number and handedness of the rotavirus inner capsid, and provided evidence for the open-mesh model, in which the five- and six-coordinated axes are represented by 'holes' formed by smaller trimeric morphological subunits.