Hamster cells, untreated and treated with chemical carcinogens, maintained in vitro for 2 1/2 years.

We have maintained in culture, for a prolonged period, untreated hamster cells from whole embryo, foetal brain and lung from newborn animals. Among the 7 lines studied we observed only one spontaneous transformation during the first year of culture. The cells of the 6 other control lines remained normal and diploid, and were not transplantable during the first 9 to 12 months of culture. After the 12th month, changes appeared in their in vitro behaviour and their transplantability: grafts of 0-5-2 X 10(6) cells induced tumours in the hamster; fewer cells did not. In vitro chemically transformed hamster cells were fundamentally different from untreated cells of the same origin, not only in morphological and growth characteristics but also in transplantability; of the 9 lines obtained, 7 induced tumours after injection of 10(1)-10(4) cells, and 2 after injection of 10(5) cells per animal.