About 200 antigenically related adenoviruses were isolated from cases of infantile diarrhoea in the Netherlands and North-West Germany. The viruses were fastidious and failed to replicate serially in human diploid fibroblasts and in primary human embryonic kidney cells. A number of strains were established in HeLa, HEp-2, Graham (293), cynomolgus monkey kidney, and Chang conjunctival cells. The viruses were mammalian adenoviruses by the usual criteria. No relationship to the 39 known human adenovirus species was found, either by neutralization tests or by haemagglutination inhibition tests. Neutralization tests showed two distinct variants, represented by strains Tak and Dugan. The variants were identical in haemagglutination inhibition tests. DNA restriction enzyme analysis showed Tak and Dugan to have considerably different genomes, indicating that these variants should be classified as different species (Wadell et al, 1983). It is proposed that the variants should be called Mastadenovirus h 40 (with reference strains Dugan and Hovi X) and Mastadenovirus h 41 (with reference strain Tak). Neutralization and haemagglutination inhibition tests demonstrated that the viruses from Glasgow and Helsinki (Hovi X) described by Johansson et al  and by Kidd and Madeley  belong to these two adenovirus species.