The incorporation of thymidine into deoxyribonucleic acid in mouse fibroblasts infected with polyoma virus.

1. Mouse-fibroblast cultures in the stationary phase of growth show an increased rate of [(3)H]thymidine incorporation into DNA from 12 to 44hr. after infection with polyoma virus. 2. Intracellular virus progeny is first detected at about 24hr. after infection. 3. Calculations based on the [(3)H]thymidine-incorporation data and direct measurements of the DNA content of the cell cultures indicate that the amount of the excess of DNA synthesized by the infected cell cultures corresponds to about 10% of their total DNA. 4. The mitotic index of the cell cultures at 40hr. after infection was significantly higher than that of non-infected control cells. 5. Possible interpretations of the stimulation of DNA synthesis observed in polyoma-infected cell cultures are discussed.