SummaryTo obtain information on inhibition of adenovirus transformation by AAV, cultures of hamster embryo cells were inoculated simultaneously with constant amounts of simian adenoviruses SA7 or SV11, or with human adenovirus type 12 and with varying multiplicities of AAV-1. Induction of transformed cell foci by SV11 or Ad12 was inhibited by more than 80% when 9 to 10 plaque inhibiting units (PIU)/cell of AAV-1 were added. The amount of AAV-1 necessary to reduce transformed cell foci by 50% ranged from 1.5 to 2.5 PIU/cell for SV11 and Ad12 to 5 to 7 PIU/cell for SA7. Analysis of the dose-response data indicates that 1 unit of AAV-1/cell is sufficient to inhibit transformation by Ad 12 and SV11. The relative resistance of SA7 transformation to inhibition seems best explained as a requirement for more than 1 inhibiting unit/cell of AAV-1.