Fanconi's anemia (FA) cells are highly susceptible to both reactive oxygen species and mitomycin C (MMC), a DNA cross-linking agent. In this study we have determined the amounts of 8-hydroxydeoxyguanosine (8OHdG), typical of oxidative DNA damage, in Epstein-Barr virus transformed lymphoblasts from FA patients and normal controls by the use of HPLC combined with electrochemical detection. FA cells (HSC72 and 99 cells being assigned to FA complementation group A) formed 2-3 times more 8OHdG than control cells after incubation with 20 mM H2O2 at 37 degrees C for 30 min. FA cells also formed more 8-hydroxyguanosine, typical of oxidative RNA damage, than control cells. FA cells showed decreased activity to decompose H2O2. Although the activity in FA cells was only 20-30% less than control cells, the remaining, undecomposed H2O2 concentration was almost twice as much in FA cells as in control cells, and the remaining H2O2 concentration correlated well with the amounts of 8OHdG formation. The H2O2 decomposing activity was almost completely inhibited by sodium azide (NaN3) or aminotriazole, both catalase inhibitors. With these inhibitors the amounts of 8OHdG formation were much higher than in those cells without inhibitors, and were almost the same in control cells as in FA cells. Catalase activity in FA cell lysates was 70-80% of controls. MMC also increased 8OHdG formation in FA cells only at ED100 but not at ED50. These results indicate that FA cells, at least FA complementation group A cells, have increased susceptibility to oxidative DNA damage, and that this increased susceptibility is possibly due to decreased catalase activity. These results also suggest that catalase plays an important role in protecting DNA from oxidative damage. However, this increased susceptibility to oxidative DNA damage is considered not to be the major cause of the increased susceptibility to MMC.