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The chicken-embryo-lethal-orphan (CELO) virus, an avian adenovirus, is frequently found as a latent agent in chickens (1,8,9). It was first isolated as a contaminant of chicken embryos (9, 10). More recently, the role of CELO virus as a possible tumorigenic agent was reported by SXRMA et al. (7). They showed that hamsters inoculated subcutaneously with the virus developed tumors. However, it was not definitely proven that CELO virus was the etiologic agent. This study was designed to establish CELO virus as a tumor-inducing agent in golden Syrian hamsters.Four criteria were used to prove CELO virus as an oncogenic agent: 1) the presence of CELO virus in the tumors or in cell cultures prepared from tumors;2) the presence of type specific CEL0 antibody in tumored hamsters inoculated with CELO virus or tumor transplants;3) the rejection of tumor cells in transplantation immunity studies; 4) the presence of type specific CELO "T" antigen in tumor cells. The Phelps strain of CELO virus was propagated in embryonated chicken eggs from hens that were free of the virus. One-to 3-day old hamsters were inoculated subcutaneously (s.c.) in the dorsal area with 0.1 ml of CELO virus stock which titered from 10 s-~ to 109.5 embryo lethal doses (ELD50/0.1 ml). Virus-induced hamster tumor tissue or cells from two cell cultures (T5 and T37) prepared from CELO-induced hamster tumors were implanted or inoculated into weanling hamsters.1. Virus Isolation Cell suspensions (10~o) from 44 tumors and pelleted concentrates from cell extracts of 15 tumors were tested for the presence of infectious CELO virus.

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