Summary All of 3 strains of type 7 ade-novirus isolated in primary human embryonic kidney cell cultures from respiratory secretions of human subjects with acute respiratory illness proved oncogenic for newborn hamsters. The type 7 viruses were isolated and propagated in human embryonic cell cultures and were free of extraneous agents. Detailed information relative to the virology, oncology, pathology and immunology of the type 7 adenovirus-hamster tumor system is presented. The tumors appeared after a long latent period, were highly malignant, were readily transplantable and were free of detectable infectious virus. The cells in most of the tumors were sufficiently differentiated to permit a pathologic diagnosis of malignant lymphoma, or lymphosarcoma when evidence of invasiveness was present. A majority of tumors induced by the virus presented a specific complement-fixing antigen which reacted with sera from hamsters bearing homologous tumor. The authors are indebted to T. Beddow, W. Clark, P. Gosnell, B. Keech and W. Raupp for technical assistance.