Triadimefon, propiconazole and myclobutanil are conazoles, an important class of agricultural and therapeutic fungicides. Triadimefon and propiconazole are mouse liver tumorigens, while myclobutanil is not. All three conazoles are generally inactive in short-term genotoxicity tests. We studied the in vivo mutagenicity of these three conazoles using the Big Blue mouse assay system. Groups of mice were fed either control diet or diet containing 1800 p.p.m. triadimefon, 2500 p.p.m. propiconazole or 2000 p.p.m. myclobutanil. After 4 days of feeding, mice were immediately euthanized, livers were removed, DNA isolated and lacI genes recovered into infectious bacteriophage lambda particles by in vitro packaging. Bacteriophage with mutations in the lacI gene was detected by infecting into Escherichia coli, and mutant frequencies were determined using a colorimetric plaque assay. Propiconazole induced a 1.97-fold increase in mutant frequency compared to concurrent controls (P = 0.018) and triadimefon induced a 1.94-fold increase compared to concurrent controls (P = 0.009). Myclobutanil did not induce any change in mutant frequency (P = 0.548). These results provide the first evidence that the hepatotumorigenic conazoles are capable of inducing mutations in liver in vivo while the non-tumorigen myclobutanil is not, suggesting that mutagenicity may represent a key event in conazoles tumorigenic mode of action.