The effect of tritiated thymidine on the rate of entry of cells into mitosis was studied in mice; 1.5 µc of tritiated thymidine (specific activity 5 c/mM) per g body weight did not alter the rate of entry. Other workers have reported an increased number of mitoses following administration of larger amounts of thymidine than those used in the present work; the suggestion is therefore made that when tritiated thymidine is given to animals for studies on cell proliferation the dose of thymidine in a single injection should be not greater than 0.1 µg/g body weight.

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