The DNA of the prototype strains of ovine adenovirus (OAdV) 1 through 5 was analysed by restriction enzyme (RE) digestion. The RE patterns generated by Hindlll and PstI enzymes were characteristic of the examined strains. OAdV-2 and 3 resembled each other the most, and their EcoRI and Hindlll patterns seemed to be identical. Considering the number of comigrating fragments, serotypes OAdV-2, 3, 4 and 5 looked more closely related to each other than to OAdV-1. This finding was strengthened by Southern blot hybridisations probed with random Hindlll clones of OAdV-3. The estimated genome size of the examined OAdV types ranged between 31.9 and 32.8 kilobase pairs. The results supported the new genus classification of OAdVs.