Percent-labeled mitosis curves were obtained on Day 4 of growth in Sarcoma 180 in vitro at different counting thresholds and at different emulsion exposure times. A comparison of these curves with those obtained on Day 2 of growth shows that there is a pronounced shift in the cell cycle time distribution over the 2-day interval. The shape of the percent-labeled mitosis curve is dependent on counting threshold and on emulsion exposure duration. The gradual rise and fall of the labeling intensity

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... of the first wave demonstrates that the onset and termination of DNA synthesis are not abrupt events in the cell cycle. At low threshold and prolonged emulsion exposure duration, low levels of DNA synthesis are demonstrable throughout much of the early portion of long cell cycles. The labeling index is also dependent on counting thresh old and emulsion exposure duration, casting doubt on the existence of a discrete nonproliferating pool. The DNA content distribution exhibits a progressively more prominent postmitotic peak with increasing popula tion size. This parameter reflects changes in the predomi nant DNA synthesis patterns that are associated with shifts in the cell cycle time distribution. Correlations between the DNA content distribution and tritiated thymidine para meters, and the role of each in the interpretation of the other, are discussed. ' To whom requests for reprints should be addressed at NIH. Building 36. Room 4D05. Bethesda. Md. 200I4. 2The abbreviation used is: PLM. percent-labeled mitosis.