In this paper it is reported that the recovery from 6—12 Gy X-ray irradiation-induced potentially lethal damage (PLD) in plateau-phase V,79, cells, following incubation for 2—6h postirradiation was observed. The survival ratio was 1.5—2.0. While using nontoxic concentration below 4 mmol/L of metronidazolamic acid (CM), the significant inhibiting effects of CM on 6—12 Gy X-ray induced PL-DR was evaluated. The repair inhibition factor (RIF) ranged from 1.31—46.30. However, the CM treatment with a 6 h incubation following 12 Gy radiation showed the highest RIF which were 14.03, 28.16 and 46.30 for 0.5, 1.0 and 2.0 mmol/L CM respectively. These results indicate that CM strongly inhibits radiation induced PLDR and the effectiveness depends on CM concentration, postirradiation incubation time and the radiation doses, suggesting that CM fixes the X-ray induced PLD, and thereafter prevents its repair. The molecular mechanism of CM inhibiting effects on PLDR is discussed.