Effect of tritiated thymidine on the kinetics and viability of 9L cells in vitro.

The colony-forming efficiency of 9L rat gliosarcoma cells was unaffected by treatment with 0.1 muCi/ml of [3H]TdR. However, when cells were treated with 1 or 10 muCi/ml of [3H]Tdr, cell growth was reduced and cell survival decreased. When monolayer 9L cells were treated with 1 muCi/ml of [3H]TdR for up to 72 hr, approximately 5% survived, which is closely related to the percentage of non-cycling cells in this system. When cells were treated with 10 muCi/ml of [3H]TdR for 72 hr, less survival was observed. The additional cell kill observed may be induced by [3H]TdR released from doomed cells into petri dishes during the incubation period of the colony-formation assay.