Cell kinetic studies on the JB‐1 ascites tumour of the mouse

. The FLM method, modified by double labelling with [ 3 H]‐ and [ 14 C]‐thymidine, has been applied to the 4‐day old JB‐1 ascites tumour of the mouse. It results in well separated waves of purely [ 3 H]‐ and purely [ 14 C]‐labelled mitoses, which show a remarkable asymmetry with long tails to the right. The following values for the mean transit times of the cells have been derived from this FLM curve, for a tumour age of 4–6 days: T C = 32.5 hr, T S = 16.7 hr, T G1 = 3.7 hr, T G1 = 11.0 hr and T M = 1.1 hr. A further evaluation of the FLM curve, however, is difficult, due to the non‐stationary growth of the tumour. A number of other experimental findings (growth curve, decrease of the labelling and mitotic index with increasing tumour age, two single‐labelled FLM curves starting 4 and 6 days after tumour inoculation) indicate that the cell cycle time increases during the experimental period of the double‐labelled FLM curve (about 2 days). A lengthening of the cycle time should result in an increasing enlargement of the areas under the waves of the modified FLM curve. However, such an increase in area has not been found; the areas are constant. All the results of the present cell kinetic studies would be consistent if it were postulated that the cell cycle time lengthens with increasing tumour age up to about 4 days after inoculation, then remains relatively constant at between 4 and 6 days and thereafter increases again. Short‐term double labelling experiments suggest that this is actually the case. Under the assumption of nearly constant phase durations during the 5th and 6th day of tumour growth further conclusions can be drawn from the modified FLM curve. In particular, it follows that the transit times of the cells through successive cycle phases are uncorrelated and the variances of the transit times through a cycle phase are proportional to the duration of this phase.