The interaction between antibodies and surface antigens of intact immobilized adenovirus particles was studied by single radial immunodiffusion tests (SRDT) in agarose gels. Purified virus particles and antisera against virus particles of selected serotypes representing different subgroups and against structural components of certain serotypes were employed. Antibodies against hexons, pentons and fibres all gave visible zones. Antisera against the latter components were more efficient in forming zones and this was though to be due to the occurrence of relatively smaller amounts of vertex capsomers and fibre antigen at the virus particle surface. The capacity of antibodies against vertex capsomers to form zones was demonstrated in tests with virus particles of intermediate types. The SRDT with immobilized virus particles was found to be highly sensitive for the demonstration of virus particle surface antigens shared between serotypes. Results obtained in previous studies using virus particles haemagglutination-inhibition (HI) tests including anti-antiserum were confirmed. Cross-reactions mainly occurred within subgroups and were ascribed to a sharing of vertex capsomere antigens. In preliminary studies a rapid typing was achieved by examination of paired human sera from five cases of adenovirus infections.