Significance of continued virus production in tissue cultures rendered neoplastic by polyoma virus

The infection with polyoma (PY) virus of monolayer cultures of either mouse or hamster embryo cells gives rise to a neoplastic transformation of the cultures.(1) Previous results showed that the transformed hamster cultures did not release any virus, whereas the transformed mouse cultures continued to release PY virus indefinitely; in the latter case, the virus released was invariably a mutant of small-plaque (sp) type.(2) We have since observed that in some cases transformed hamster cultures also continue to release virus indefinitely and that the virus released is also of the sp type.