LABELING OF MURINE MASTOCYTOMA CELLS IN VITRO WITH PLASMA TRITIATED THYMIDINE-LABELED ANIMALS

40 min after injecting tritiated thymidine into an animal, 20–30% of the total plasma radioactivity is nonvolatile. This fraction decreases to about 6% 10 hr after the injection and 3% 24 hr after the injection. There appears to be material in this nonvolatile fraction that can label mastocytoma cells in culture. The labeling indices decrease with time after injection in the same way as the nonvolatile fraction. The 40 min plasma sample contains sufficient material to allow accurate assessment of the fraction of cells in S in culture after a 6 wk exposure. The circulating material is not apparently available for incorporation into those cells in cycle in the donor animal. The material appears to be related to the G₀ cell-specific pool that has been described elsewhere. The trichloroacetic acid-soluble or ethanol-soluble nonvolatile activity appears to contain thymine, and some thymidine-phosphorylated compounds.