Comparative hemagglutination-enhancement (HE) tests demonstrated diversified patterns of antigenic specificities both in the fiber and vertex capsomer part of pentons of human adenovirus types 3, 11 (subgroup I), 9, 15 (II), 1, 2, 4, 5, 6 (III), and 12. All fibers contained a type-specific antigen. Subgroup II and III fibers, in addition, contained specificities both unique for each subgroup and also common to the two subgroups. Fibers of serotypes 4 and 12 displayed a somewhat deviating behavior. All vertex capsomers tested shared a group-specific part. This was the only antigenic specificity demonstrable for serotype 12. Maximal penton HE titers of all sera were reached in tests with incomplete hemagglutinin of type 11. In addition, maximal HE activity of sera against individual serotypes also was recorded against pentons of other members of the same subgroup. Antigen characteristics of vertex capsomers of type 4 indicated a closer relationship to subgroup I than to subgroup III. The toxin activity of pentons was more sensitive to trypsin treatment than their capacity to function as incomplete hemagglutinin. Homotypic antipenton sera, unabsorbed or absorbed with homotypic fibers to remove antibodies against this component, and, to a varying extent, also heterotypic antipenton sera could neutralize toxin activity. Antifiber sera could neutralize toxin activity of pentons carrying short fibers (10 nm, type 3) but not of those carrying long fibers (28 to 31 nm, type 2). It is concluded that toxin activity is carried by a specific part of vertex capsomers and that cell detachment can be brought about via a direct contact between this component and cell membranes. Fiber-mediated attachment does not seem to be necessary for this biological activity to become expressed.