Much indirect evidence supports the assumption that tritiated thymidine does not label cells which enter the deoxyribonucleic acid synthesis phase (S) more than 1 h after injection. Direct evidence confirming this assumption was obtained by counting labelled epithelial nuclei in mice killed 1, 4 or 6 h after a single intraperitoneal injection of [3H]thymidine; colchicine was used to prevent the increase in number of labelled nuclei which would otherwise have occurred because of cell division. The proportion of cells labelled was the same at 1 h as at 4 or 6 h after injection of [3H]thymidine. Nuclei were regarded as labelled if they were overlaid by 4 grains or more; comparison of nuclear and background labelling indicated that nuclei overlaid by 3 grains or less represented background labelling.

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