Antigenic relationships between adenoviruses of subgenus D were determined by neutralization tests in HeLa cell cultures by CPE inhibition. For cross-testing, several antisera of the same species were tested against the prototype viruses. 39 wild strains belonging to 12 different virus species were also studied. Marked variation in the degree of cross-neutralization between individual sera of the same species was often observed. However, virus strains within a species mostly showed identical serological reactions. Hence, antigenic specificity appears to be a fairly constant property of any one species. Strong cross-neutralizations between species are presumably due to a relationship of the hexon (hexon) antigen, whereas weak cross-neutralizations found between viruses related by hemagglutination-inhibition are due to the gamma (fiber) antigen. Viruses related to adenovirus 15 (Mastadenovirus h 15) showed a variety of cross-reactions in neutralization tests. In view of the new species definitions of adenoviruses and to facilitate identification, changes in the classification of Ad 15, 25, 29, and 15/H9 are proposed. The prototypes of Ad 13, 15, 25, 29, and 30 have been cloned by terminal dilution.