The adenovirus fibre carries the type-specific gamma determinant the existence of which was suggested by haemagglutination inhibition tests. Furthermore, the fibre is thought to be responsible for the agglutination of monkey, rat and human erythrocytes. In order to verify that the haemagglutination properties and the type-specific gamma determinant are located on the fibre knob domain, several recombinant fibre proteins of subgenus D adenoviruses were constructed, expressed in HeLa cells and tested in haemagglutination and haemagglutination inhibition tests. Our data showed that the epitopes responsible for the interaction with erythrocytes and the gamma determinant are located on the fibre knob domain.