Seven chemicals, six of which are known to induce epithelial neoplasms of the urinary bladder in rats, were assayed for their ability to induce DNA damage in primary cultures of rat and human cells from urinary bladder mucosa, and in urinary bladder, liver and kidney of intact rats. Significant dose-dependent increases of DNA fragmentation, as measured by the Comet assay, were obtained in cells from both rats and humans with the following concentrations of five test compounds: 2-naphthylamine and N-nitrosodi-n-butylamine 0.5 and 1 mM, phenacetin 2 and 4 mM, cyclophosphamide from 2 to 8 mM, and o-toluidine 16 and 32 mM. Nitrilotriacetic acid (1-4 mM), a rat bladder carcinogen, and 4-aminobiphenyl (0.125-0.5 mM), a bladder carcinogen in humans but not in rats, gave a weak positive response in rats cells and a more marked response in humans cells. In terms of DNA-damaging potency, 4-aminobiphenyl, cyclophosphamide, phenacetin and 4 nitrilotriacetic acid were more active in human than in rat cells, whereas the converse occurred with 2-naphthylamine. Consistently with the results observed in vitro statistically significant dose-dependent increases in the average frequency of DNA breaks were detected in the urinary bladder mucosa of rats given p.o. single doses corresponding to 14 and 12 LD50 of six of the seven test compounds; the only one which gave a substantially negative response was 4-aminobiphenyl. With the exception of N-nitrosodi-n-butylamine which caused DNA damage in liver and of phenacetin and nitrilotriacetic acid which caused damage in kidney in agreement with their tumorigenic activity, any substantial evidence of DNA lesions in these two organs was absent in rats treated with 12 LD50 of the other 4 test compounds. These findings give evidence that urinary bladder genotoxic carcinogens may be identified by the DNA damage/Comet assay using as targets cells of urinary bladder mucosa, and show that the effect may be quantitatively different in cells from rats and from human donors.