It was shown that the administration of zinc-containing yeast (zinc-yeast) to the abdominal cavity significantly increased the survival of mice irradiated with 7.5 Gy X-rays (LD50/30 assay result). In order to elucidate the molecular mechanism of this radio-protection effect, we examined TK6 (p53 wild type) and WTK1 (p53 mutant) lymphoblast cell lines exposed to 0.1mg/ml zinc-yeast. In TK6 cells, the cell growth was impaired after administration of zinc-yeast and the cells seem to accumulate at S/G2 phase in the cell cycle. On the other hand, in WTK1 cells, no effect on cell growth was observed. Furthermore, we measured the occurrence of apoptosis in X-irradiated cells (10 Gy) treated with zinc-yeast post-irradiation. In TK6 cells, a significant reduction in apoptosis was observed with the combined treatment, while there was no clear difference observed in WTK1 cells under the same treatment. These data indicate that the radio-protective effect induced by zinc-yeast might be affected by the p53 status of irradiated cells.13th International Congress of Radiation Researc