AbstractThe proliferation kinetics of the transplanted adenocarcinoma 284 of C3H mice from the first to the sixth day after 600 R and from the first to the eighth day after 1,200 R has been studied by repeated labelling of the tumour cellsin vivowith3H-thymidine and measurement of the labelling index and the percentage of labelled mitotic cells. The time course of the post-irradiation synchronization was followed during the first day. Later, the mean generation times of the tumour cells were usually prolonged and the spread of the generation times increased. Three to four days after irradiation, the tumour increased its growth fraction to twice the normal value by triggering resting cells (G0cells) into cycle.