The preposed studies extend previous work aimed at outlining the neuro-oncogenicity of JC virus, a human polyoma (DNA) virus, for its most susceptible host, the Syrian hamster (Mesocricetus auratus). Several isolates of this virus have been obtained from the brains of human adults who had suffered from the subacute demyelinating disease called progressive multifocal leukoencephalopathy. Two of these isolates will be used since their oncgenic effects have been found to vary. Tumors induced so far have all been malignant and several of these resemble most closely central or peripheral nervous system tumors seen in childhood or infancy, sometimes even on acognenital basis. By injecting JC viral preparation at different stages of hamster development, we intend to determine the susceptibility for tumor induction especially in cerebellar cortical nerve cells, at periods other than the neonatal. The latter had been tested previously. The oncogenic effect of JC virus on the hamster retina will be studied utilizing two different strains and tumors in the hypothalamic-pituitary area will be analyzed. The central nervous system of two owl monkeys which developed brain tumors after JC viral inoculations will be studed.