1,8-Dihydroxyanthraquinones are under debate as plant-derived carcinogens that are found in laxatives, food colors, and possibly vegetables. Published genotoxicity data are controversial, and so three of them (emodin, danthron and aloe-emodin) were tested in a number of in vitro assay systems. All three compounds induced tk-mutations in mouse lymphoma L5178Y cells. Induction of micronuclei also occurred in the same cell line, and was dose-dependent, with the potency ranking being danthron > aloe-emodin > emodin. In a DNA decatenation assay with a network of mitochondrial DNA of C. fasciulata, all three test compounds inhibited the topoisomerase II-mediated decatenation. Danthron and aloe-emodin, but not emodin, increased the fraction of DNA moving into comet tails when tested at concentrations around 50 microM in single-cell gel-electrophoresis assays (SCGE; comet assay). Comet assays were also used in modified form to determine whether pretreatment of the cells with the test compounds would reduce the effects of etoposide, a potent topoisomerase II inhibitor. All three test chemicals were effective in this pretreatment protocol, with danthron again being the most potent. Given clearcut evidence of their genotoxic activity, further research on the human cancer risk of these compounds may be warranted.