As an animal model to study the etiological significance of the adenoviruses isolated from human tumors or cancer patients, adenovirus type 12 (ad-12), an oncogenic type, and type 6 (ad-6), a non-oncogenic type, were inoculated into hamsters simultaneously or at some intervals, and the recovery of viruses from tumors and the liver was tried. From tumors, neither ad-12, the causative agent of tumors, nor ad-6, non-causative agent, was recovered. From the liver, only ad-6 was recovered. It thus appears that, in hamsters, ad-12-induced tumors yield neither the causative nor non-causative "passenger" adenovirus, though tissues other than tumors might yield non-causative "passenger" adenovirus. When inoculated simultaneously with or 2 days after ad-12, ad-6 inhibited the tumor induction by ad-12. This inhibitory effect was not observed when ad-6 was inoculated 4 or more days after ad-12, suggesting that this inhibitory effect of ad-6 would be due to its cytocidal effect on the target cells of ad-12 and that, once transformed by ad-12, cells become resistant to ad-6 and form tumors.