ICOSAHEDRAL capsids of adenoviruses are each composed of 252 capsomeres1 of which 240—situated at the faces and sides of the triangular facets—are called hexons, and the remaining 12—situated at the vertices—are called pentons2,3. The hexons are approximately spherical, with a diameter of 80 A, whereas the pentons consist of a base which is morphologically similar to the hexons, to which is attached a fibre with a terminal knob. Hexons, pentons and fibres are found as free antigens in extracts of infected cells and can be purified by a variety of techniques. The study of their morphological, physicochemical, antigenic and biological properties has received a great deal of attention (see review by Pereira and Valentine4). None of these antigens (or, indeed, the antigens of any other animal virus) has previously been crystallized.