Summary The relationship of group-specific (hexon) antigen, type-specific (fibre) antigen, soluble haemagglutinin (dodecon) and purified virus particles of adenovirus 19 was studied by cross complement-fixation, haemagglutination- inhibition and neutralization with antisera prepared against purified antigens. The fibre antigen reacted as an indirect haemagglutinin, producing haemag- glutination in the presence of selected heterologous adenovirus immune sera. Dodecon and fibre antigen were closely related, while hexon antigen showed no relationship to fibre and possibly a weak relationship to dodecon antigen. Neutralizing antibodies were found in antisera to hexon and dodecon, but not in fibre antisera.