Cell cycle parameters measured by the percentage labelled mitoses (PLM) technique have been compared with the results of grain counting and continuous labelling studies in the mouse tumour Carcinoma NT. In [3H]deoxyuridine( [3H]UdR)-labelled tumours a PLM curve showed that the mean length of DNA synthesis (TS) was 7 to 9 hr and the mean cell cycle time (TC) was 16 hr. The turnover time (TT) was between 13 and 17 hr. In contrast, the rate of grain count halving showed that mean TC was 40 hr and TS was at least 19 hr. Cells entered S phase at a rate of 2.3%/hr with continuous labelling, which gave values of 22 hr for TS and 39 to 44 hr for TT. The PLM data therefore gave mean kinetic parameters which were less than half the lengths shown by the other techniques. We conclude that in Carcinoma NT there is a wide range of cell cycle times. The grain count halving and continuous labelling data showed the true mean values of TS and TC or TT, while the PLM data showed only the fastest cycle times. Therefore in this tumour, calculating the cell loss factor from PLM data would give a value too high by at least a factor of two.