Two systems utilizing extracts derived from nuclei of adenovirus-infected cells which synthesize adenovirus DNA in vitro were analysed for indications of ADP-ribosylation of virus proteins. On incubation with [32P]NAD or [14C]NAD, modification of the adenovirus T antigen could be demonstrated in one of these systems. ADP-ribosylation of adenovirus core proteins V and VII could also be demonstrated with both nuclear extracts. However, using 3-aminobenzamide, a specific inhibitor of poly (ADP-ribose) polymerase, there was no evidence either in vivo or in vitro that ADP-ribosylation played a critical role in the replication process.