THE adenoviruses comprise a group of icosahedral animal viruses1,2 with two types of capsid proteins. One of these, called the penton, occupies the five-fold positions at the twelve corners of the icosahedron. The other, called the hexon, occupies the six-fold positions on the faces and edges of the icosahedron, there being 240 hexons on the surface of each virus particle. The hexon antigen contains both group specificity, demonstrated by complement fixation and gel diffusion, and type specificity demonstrated by its ability to induce type-specific neutralizing antibody3.