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Values of Ts provided by the double labelling method have been compared with those given by the percentage labelled mitoses curve for blast cells in the peripheral blood of a patient with plasma cell leukaemia and of rats bearing a transferable acute leukaemia. the double labelling method was carried out giving the first label (3H‐thymidine) in vivo and the second label (14C‐thymidine) in vitro with several values for the interval between the two labels. Ts was calculated by fitting regression lines to the results obtained. Data for percentage labelled mitoses were analysed by computer. For the plasma cell leukaemia values of Ts= 17.1 ± 7.0 hr and Ts= 19.8 ± 3.4 hr, and for the rat leukaemia values of 8.7 ± 1.7 hr and 9.0 ± 1.7 hr (7.1 hr corrected for exponential growth) were obtained from the percentage labelled mitoses and double labelling methods respectively. It is concluded that the double labelling method is valid for the study of cell proliferation in leukaemic blast cells.