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The vacuolating agent, SV40, was first discovered as an indigenous contaminant of primary cell cultures of rhesus and cynomolgus fied at 2000 rpm for 20 minutes in the horizontal centrifuge. The supernate used in the animal experiments titered 1 k 6 per 0.2 ml in monkey kidney and of oral poliovirus vaccine ( 1,2). Subsequent findings revealed an oncogenic quality of the virus with induction of fibrosarcomatous neoplasms and ependymomas(3-5) in newborn hamsters and of papillary ependymomas (6) in newborn Mastomys. The oncogenic quality of the virus has become more understandable in the recent demonstration of its close biologic relationship to other tumorigenic agents including papilloma viruses of man and rabbits and of the mouse polyoma agent ( 7 ) . The virus produces subclinical infection in human subjects following respiratory inoculation or oral ingestion and is excreted into the feces subsequent to inoculation by the latter route (8,9). In primary embryonic, neonatal or adult human cells in culture, the virus causes epithelioid transformation with consistent chromosomal aberration of a probable heritable nature ( 10-13).The importance of SV40 virus as a malignant oncogenic agent prompted descriptive studies of the pathogenesis of the virus in the hamster model. The present report presents findings in investigations to measure the effect of host age, virus dose, and route of inoculation on oncogenic efficiency and to ascertain the transmissibility of the agent by contact.Materials and methods. The detailed methods described previously (3) were generally followed. Virus strain VA 45-54 derived from uninoculated grivet monkey kidney (GMK) cell culture was used. Virus in first subculture passage was employed and exhaustive tests revealed that Sv40 was the only detectable agent present. The infected cell cultures were frozen and thawed several times at time of harvest and the virus pool was clari-*This work was supported in part by grant from U. S. Public Health Service. GMK cell cultures (10 day reading) and was stored frozen in flame-sealed glass ampules in the dry ice refrigerator. Control tissue culture fluid (NTCF) was prepared in similar manner from uninoculated cell cultures of the same lot of GMK and was stored as described above. Control medium (CM), composed of medium 199 containing 2 70 of heat-inactivated chicken serum, p H 7.0, was the same lot used for maintenance of the cell cultures described above and was stored in similar fashion. Hamsters were from the Lakeview Hamster Colony, Lakeview, N. J., and were inoculated when newborn or 7 , 30 or 90 days of age. The virus dose was 0.2 ml given subcutaneously unless otherwise indicated. The virus was titrated for infectivity in GMK each time it was used. When diluted for hamster inoculation, medium CM was employed. Animal care, clinical observations, and pathologic examinations were as described previously. Serology. Hamster sera were tested for neutralizing antibody using 30-300 TCDS0 of strain 45-54 of SV4, virus. The sera were inactivated at 56°C f...