This study was aimed at identifying the roles of caffeine and acriflavine, two repair inhibitors, on UV sensitivity of iron-oxidizing Thiobacillus ferrooxidans ATCC 13728. The UV-dose response survival curve was inflected in nature, suggesting the population heterogeneity of the isolate. Caffeine and acriflavine potentiated the UV-induced killing of the organism. With the increase in concentrations of these compounds. the extent of survival decreased. Similarly, the inhibitory effects of caffeine and acriflavine increased with the increase in dose of UV-irradiation. The cells irradiated with 10 s (equivalent to 5.6 x 10(-5) J/m(2)/s) of UV-exposure tended to become resistant to the inhibitory effects of caffeine and acriflavine, as evidenced by the time course study of recovery. The cells appear to stage a dramatic recovery from UV damage in the presence of caffeine (3.0 mg/ml) and acriflavine (20 mug/ml) over a period of 25-30h and 35-40h respectively, when grown in the presence of energy sources.