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A virus was isolated from a radioresistant feline fibrosarcoma. It induced multinucleated giant-cell formation and lysis in a cell line derived from a canine fibrosarcoma, which was used to characterize the virus. End-point titrations in these cells required 28 days. The virus was sensitive to ether and heat and was destroyed at pH 3. Replication was not inhibited by 5-bromodeoxyuridine. Electron microscopy revealed assembly by a budding process from the plasma membrane of infected cells. The average diameter of the virion was 106 nm. Intracisternal particles with an average diameter of 45 nm were present within infected cells. In two instances secondary monolayers of feline renal cells underwent morphological transformation after inoculation of the virus. The two strains of transformed cells are now in continuous culture and do not yield infectious virus. MATERIALS AND METHODS Virus. The pool of virus used in these studies was prepared from the third subpassage of the virus in a cell line derived from a canine fibrosarcoma. Cellculture medium and infected cells were harvested 12 days postinfection. The pool was frozen at-90 C and subsequently thawed, redistributed in portions, refrozen, and maintained in liquid nitrogen vapor. This pool of virus was titrated 27 times over an interval of 27 months. The average titer was 10155 50% tissue culture infectious doses (TCIDso) /ml. The infectivity titer did not decrease during storage. Canine herpesvirus (CHV) was used as a control deoxyribonucleic (DNA) virus in studies of the effect of 5-bromodeoxyuridine on virus replication. CHV was obtained from