The main antigen and immunogen of canine adenovirus type 1 (CAV-1) has been purified to near homogeneity from cultural fluid of a CAV-1-infected primary cell culture by hydrophobic and anion-exchange chromatography. The hexon native form (trimer) was shown to be resistant against denaturation by SDS under conditions of SDS-PAGE performed without heating the samples. The monomer chain of the CAV-1 hexon was apparently identical in terms of electrophoretic mobility with that of the previously sequenced BAV-3 hexon polypeptide (103 kDA). In blot enzyme immunoassay only native trimers of CAV-1 hexon were detected by cross-specific polyclonal and monoclonal anti-hexon antibodies.