The effect of 12C6+-ion irradiation on the survival of Clostridium tyrobutyricum ATCC 25755. The cells were irradiated 20 h after seeding and the extent of survival was determined by MTT assay. Survival data is plotted against the irradiation levels. (A-C) 12C6+-ions were accelerated up to 68 AMeV, and their ions/pulse ranged from 106 to 108, at a dose rate of 10 to 50 Gy. (D-F) 12C6+-ions were accelerated up to 114 AMeV, and their ions/pulse ranged from 106 to 108, at a dose rate of 10 to 50 Gy. Cells with low metabolic activity and slow proliferation or cells that cease to proliferate were excluded from the assay by washing and trypsinization when the plating was done after irradiation.