Summary Adenovirus types 13, 15, and 19 and their soluble antigens were studied by electron microscopy, applying the negative staining technique to purified material. Virus particles showed projections of 160 to 190 Å length with a final knob arising from the 12 vertices of the icosahedron. Incomplete particles were also seen. The group-specific antigen consisted of free (‘hexon’) capsomeres. The type-specific antigen, which showed the phenomenon of haemagglutination in the presence of a heterologous immune serum, had the appearance of fibres with final knobs (‘fibre antigen’). The soluble haemagglutinin appeared as a star with a diameter of 450 to 600 Å. It was composed of presumably 12 dumb-bell-shaped subunits, arranged in a regular fashion. The virus types studied were morphologically more closely related to type 3 (group I) than to type 5 (group III).