The length of the cell cycle and its phases and the growth fraction were studied in vivo in 3 transplantable rat hepatomas: a fast growing one (5123 tc), one of intermediate growth rate (7794A), and a slowly growing one (7793). The length of the cell cycle and the growth fraction for each of these tumors were found to be: 27 hr and 0.8 for 5123 tc, 49 hr and 0.42 for 7794A, and 234 hr and 0.4 for 7793. However, the values for 7793 are open to a considerable degree of uncertainty. The cell cycle times of each transplantable hepatoma were substantially similar whether a tumor was grown in the right or left thigh. Each tumor showed marked variations in the cell cycle of individual tumor cells, especially in the G2 period. For instance, the length of G2 in hepatoma 7793 varied from 5 to 40 hr.

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Supported by USPHS Research Grants CA-08373, CA-10729, and CA-07174 from the National Cancer Institute.

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