Type-2 adenovirus was shown to inhibit the growth of transplantable hamster melanoma in 70% of Syrian hamsters without any injurious effect to the host. Greatest inhibition of tumor formation was seen when animals were injected with 10(6) TCD50 of adenovirus and 2.5 x 10(5) tumor cells, or 10(6) TCD50 of virus and 5.0 x 10(5) tumor cells followed either 1 or 7 days later by a second injection of a similar dose of virus. Significant inhibition in tumor growth was also noted when 2 injections of virus (10(6.2) TCD50/injection) were given on 2 separate occasions as late as 7 and 10 days after the inoculation of tumor cells. The mechanism of tumor inhibition is not known but it could be due to a combination of factors such as viral toxicity, viral oncolysis, and antitumor immunity.