The in vitro uptake of tritiated thymidine by tumor cells was studied in carcinomas of the colon from 5 patients. There was considerable variation in the degree of tritium labeling among the 5 tumors and in different anatomic areas of the same tumor. The labeling index ranged from 3 to 47%. In general, the labeling index was higher than that reported by other investigators using the in vitro technic. It was comparable to the results obtained with in vivo uptake of tritiated thymidine of similar tumors.

The generation time has been estimated for the cells of the 5 tumors. This ranged from 22 to 155 hr for the 2 different kinetic hypotheses used.

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This investigation was supported by USPHS Training Grant 5T1 GM-380 from the Division of General Medical Sciences and by U. S. Atomic Energy Commission Contract AT(30-1)-901 with the New England Deaconess Hospital (NYO-901-31).

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