The percentage labelled mitoses technique shows the mean cell cycle time to be half its true value in Carcinoma NT. I. [3H]thymidine and vincristine studies.

Cell kinetic parameters measured by the percentage labelled mitoses (PLM) technique have been compared with those derived from continuous labelling and stathmokinetic data in the mouse tumour Carcinoma NT. In [3H]TdR labelled tumours, a PLM curve showed the length of DNA synthesis (TS) to be 9 hr. With continuous labelling, 2.5% of cells entered S phase per hour and TS was 16 hr. Six per cent of cells incorporated [3H]TdR between 1 and 3 hr after an injection of the label. The stathmokinetic technique showed that the rate of entry to mitosis was 2.0% hr. The turnover time (TT) of the population was found to be 16 hr with the PLM data and 35 and 37 hr with the continuous labelling and stathmokinetic data, respectively. The PLM technique therefore showed the mean cell cycle parameters to be about half the values obtained with other methods of measurement. We conclude that in Carcinoma NT there is a wide range of cell cycle times. The continuous labelling and stathmokinetic data represent the mean cell cycle parameters, while the PLM data give the minimum values. A cell loss factor calculated from the PLM data would, therefore, be too high by a factor of two.