Summary Previous and present findings demonstrated the failure of tumor induction in newborn hamsters by adenovirus 1, 2, 3 (tentative), 4, 5, and 6 (tentative), vaccinia, herpes simplex, varicella, cytomegalo-virus, reovirus 1 (tentative), poliovirus 1 and 3, ECHO 25, Coxsackie A10 and A21, rhino virus 11, 23 and 30, encephalo-myocarditis, chikungunya, Hawaiian dengue, Marituba, Sicilian sandfly, influenza A2, parainfluenza 1, 2 and 3, respiratory syn-cytial, mumps, measles (virulent and attenuated), rubella, rabies, visceral lymphomatosis (including RPL-12 and RIF agent), pneumonia virus of mice and lymphocytic chorio-meningitis viruses and two Mycoplasma species including Eaton agent. Adenovirus 7 was moderately and adenovirus 12 and 18 were highly oncogenic. Tumors occurred in 2 animals which Received Bryan strain Rous sarcoma. The findings are discussed in relation to the familial, biophysical, biochemical and structural properties of the viruses studied and with respect to the evidences which suggest a possible role of virus in human neo-plasia. The authors are indebted to W. Raupp, T. Bed-dow, R. Grady and W. Clark for valuable technical assistance.