Leucomalachite green is a persistent and prevalent metabolite of malachite green, a triphenylmethane dye that has been used widely as an antifungal agent in the fish industry. Concern over the use of malachite green is due to the potential for consumer exposure, evidence suggestive of tumor promotion in rodent liver, and suspicion of carcinogenicity based on structure-activity relationships. Our previous study indicated that feeding rodents malachite or leucomalachite green resulted in a dose-related increase in liver DNA adducts, and that, in general, exposure to leucomalachite green caused an increase in the number and severity of changes greater than was observed following exposure to malachite green. To characterize better the genotoxicity of leucomalachite green, female Big Blue rats were fed leucomalachite green at doses of 0, 9, 27, 91, 272, or 543 ppm for up to 32 weeks. The livers were analyzed for lacI mutations at 4, 16, and 32 weeks and DNA adducts at 4 weeks. Using a 32P-postlabeling assay, we observed a dose-related DNA adduct in the livers of rats fed 91, 272, and 543 ppm leucomalachite green. A approximately 3-fold increase in lacI mutant frequency was found in the livers of rats fed 543 ppm leucomalachite green for 16 weeks, but significant increases in mutant frequencies were not found for any of the other doses or time points assayed. We also conducted 2-year tumorigenesis bioassays in female and male F344 rats using 0, 91, 272, and 543 ppm leucomalachite green. Preliminary results indicate an increasing dose trend in lung adenomas in male rats treated with leucomalachite green, but no increase in the incidence of liver tumors in either sex of rat. These results suggest that the DNA adduct formed in the livers of rats fed leucomalachite green has little mutagenic or carcinogenic consequence.