Conditioned medium from plateau-phase cells. Effect on growth of proliferative cells and on repair of potentially lethal radiation damage.

In vitro cell cultures grown into plateau phase may be more suitable than exponentially growing cells as a model for tumor cell growth in vivo. Conditioned medium extracted from cells in plateau phase inhibits growth of log-phase cultures of either the same or a different cell line. Attempts to demonstrate that this inhibition is due to a heat-labile diffusible substance have failed. Conditioned medium extracted from plateau-phase cultures was effective in enhancing repair of potentially lethal damage following x irradiation. Cell survival is significantly increased following a six-hour postirradiation culture in conditioned medium as opposed to complete fresh medium.