Some discussion has centered on whether fowl adenovirus (FAV) serotypes 4 and 10 are distinct serotypes or in fact should be reclassified as a single serotype. We have undertaken a detailed characterisation of representatives of both serotypes in order to determine if types 4 and 10 should be grouped together or retained as distinct serotypes. Examination at the genomic level has revealed considerable similarities and few differences between these 2 serotypes. DNA cross-hybridization failed to distinguish between them and restriction enzyme analysis demonstrated limited sequence differences. In vivo studies demonstrated the cross-protection afforded by a natural route vaccination with serotype 4 FAV when chickens were challenged with serotype 10 FAV. On the basis of these studies it is suggested that these FAV serotypes be combined in future FAV classification. Physical maps for both serotype 4 and 10 have been constructed using the restriction enzymes Hpa I, Dra I, Nde I, Xba I and Not I for both serotypes and in addition Eco RI, Sfi I, Sma I and BglII for serotype 10.