complement-fixation tests failed to demonstrate antibody in the sera of tumor-bearing hamsters to autologous tumor extracts or to chicken embryo lethal orphan virus-infected chick and hamster embryo cells. INTRODUCTION CELO2 virus was isolated from eggs of apparently normal

The oncogenicity of chicken embryo lethal orphan virus for newborn hamsters has been confirmed. Tumor incidence was found to be a function of the titer of the virus inoculum. Fifty-four % (7/13) of the hamsters given i0@ 50% lethal dose developed tumors, while only 8% (8/99) of the hamsters given i0@ @ 50% lethal dose developed tumors. With a single exception, lower virus doses were nononco genic; one tumor developed among 26 hamsters given virus inactivated from 108 .7 to [email protected] 50% lethal dose by nitrous acid. Although all the tumors were of mesenchymal origin a variety of sarcomatous types were noted. The latent period varied from 14 to 52 weeks and all but one tumor appeared at the site of injection. Female hamsters appeared more susceptible than males. Indirect immunofluorescence and complement-fixation tests failed to demonstrate antibody in the sera of tumor-bearing hamsters to autologous tumor extracts or to chicken embryo lethal orphan virus-infected chick and hamster embryo cells.