The specific reaction between the various H-viruses and their homologous antibodies can be recognized in the electron microscope by the formation of aggregates which stick to the grid surface; cellular debris which does not react with antibody can be removed by washing. Under proper conditions, the aggregated virus particles tend to be linked by fibrous strands, interpreted as antibody molecules, and to be arranged in linear chains in a two-dimensional manner on the grid. In such preparations, the fine structure of the individual particles can be studied readily at high magnifications. The data presented suggest that the basic capsid architecture for all H-viruses is an arrangement of 32 capsomeres in icosahedral symmetry. There appears to be some difference in size between the various H-virus types. H-1 and HT which belong to the same serotype have an average diameter of 215 Å when the particle is measured from side to side rather than from vertex to vertex of the polyhedron. H-3 and RV, on the other hand, which are serologically related to one another, and HB, which is serologically distinct from any of the other types, are somewhat smaller and average 190 Å in diameter when measured similarly.