Treatment, for 1 h, of cultured Chinese hamster V79 cells, human liver HepG2 cells, and human colon HT-29 cells with theAlternaria toxins alternariol (AOH) and alternariol methyl ether (AME) caused a concentration-dependent induction of DNA strand breaks at concentrations ranging from 5 to 50 micromolar. After treatment for 24 h, DNA strand breaks were observed in HepG2 but not HT-29 cells. Analysis of the 24 h-incubation media of HT-29 cells showed that both toxins were completely conjugated, whereas 75% were still present as unconjugated compounds in the 24 h-media of HepG2 cells. Lysates of both cell types fortified with UDPGA were found to convert both toxins into two glucuronides each, but HT-29 cells exhibited a much high activity than HepG2 cells and gave rise to a different ratio of glucuronides. It is concluded that glucuronidation eliminates the DNA strandbreaking potential of AOH and AME, and that the two glucuronides of eachAlternaria toxin are generated by different UGT isoforms.