Crystalline silica has been classified as a human carcinogen, but there is still considerable controversy regarding its fibrogenic and carcinogenic potential. In the present study, we investigated the genotoxic potential of bentonite particles (diameter < 10 microm) with an a-quartz content of up to 6% and different chemical modifications (alkaline, acidic, organic). Human lung fibroblasts (IMR90) were incubated for 36 h, 48 h, or 72 h with bentonite particles in concentrations ranging from 1 to 15 microg/cm2. Genotoxicity was assessed using the micronucleus (MN) assay and kinetochore analysis. The generation of reactive oxygen species (ROS) caused by bentonite particles via Fenton-like mechanisms was measured acellularly using electron spin resonance (ESR) technique and intracellularly by applying an iron chelator. Our results show that bentonite-induced genotoxic effects in human lung fibroblasts are weak. The formation of micronuclei was only slightly increased after exposure of IMR90 cells to an acidic sample of bentonite dust with a quartz content of 4-5% for 36 h (15 microg/cm2), 48 h (5 microg/cm2), and 72 h (1 microg/cm2), to an alkaline sample with a quartz content of 5% for 48 h and 72 h (15 microg/cm2), and to an acidic bentonite sample with 1% quartz for 72 h (1 microg/cm2). Native (untreated) and organic activated bentonite particles did not show genotoxic effects in most of the experiments. Also, bentonite particles with a quartz content < 1% were negative in the micronucleus assay. Generation of ROS measured by ESR was dependent on the content of transition metals in the sample but not on the quartz content or the chemical modification. Reduction of MN after addition of the iron chelator 2,2'-dipyridyl showed that ROS formation also occurs intracellularly. Altogether, we conclude that the genotoxic potential of bentonite particles is generally low but can be altered by the content of quartz and available transition metals.