The cellular proliferation kinetics of an experimental fibrosarcoma in C3H mice have been studied in vitro and in vivo at various stages in tumor growth.

The duration of the cell cycle measured in vitro is the same as that measured in vivo and does not change when the increase in cell number, at first exponential, slows progressively. The slowing down of the growth rate and the plateau in vitro are explained mainly by a reduction in the proportion of cells engaged in the cell cycle and by increasing cell death.

In vivo, the growth rate is at first rapid and then slows progressively. The duration of the cell cycle is similar in all phases of tumor growth. A diminution both of the number of labeled cells after multiple injections of tritiated thymidine and of the growth fraction is seen as the growth rate slows. It is probable that in this case also increasing cell death contributes to the slowing of tumor growth. Autoradiographs in large tumors after multiple injections show considerable heterogeneity in labeling from one region of the tumor to another.

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