The virion polypeptide compositions of 15 human adenovirus types, representing all members of subgroups B and C and selected members of subgroups A and D, have been analyzed by SDS-PAGE. The capsid polypeptides II, III, IIIa and IV displayed a pattern of apparent molecular weights which was characteristic for each adenovirus serotype analyzed. Adenovirus serotypes belonging to the same subgroup were also demonstrated to share an apparent molecular weight pattern of the internal structural polypeptides in the following manner: Subgroup A:V-resolved into two bands of 46--48.5 K and 51--51.5 K; VI-25.5 K; and VII-18 K. Subgroup B:V-53.5--54.5 K; VI-24 K; and VII-18 K. Subgroup C:V-48.5 K; VI-24 K; and VII-18.5 K. Subgroup D:V-50.5 K; VI-23.5 K; and VII-18 K. Adenovirus type 4: V-48 K; VI-24.5 K; and VII-18 K. The structural polypeptides represent a major portion of the adenovirus gene products. It is therefore proposed that the apparent molecular weight differences of viral polypeptides be used (i) to confirm the identification of an adenovirus type, and (ii) for subgroup classification of adenoviruses and, consequently, that adenovirus type 4 should be classified as a member of a separate subgroup designated E.