Allyl isothiocyanate, a naturally occurring compound, component of oil of mustard and human food plants such as cabbage, cauliflower and horseradish, has up to now been regarded as nonmutagenic in bacterial mutagenicity testing systems. Recently, however, it was found to cause transitional-cell papillomas in the urinary bladder of male F344 rats. Contrary to earlier reports, in this study allyl isothiocyanate showed clear mutagenicity for Salmonella typhimurium TA100 in the preincubation assay after longer, non-standard preincubation times (greater than 20 min). The mutagenicity is expressed only in the presence of a rat-liver homogenate metabolising system, i.e. it is indirect. However, high concentrations of rat-liver homogenate suppress the mutagenicity of allyl isothiocyanate. SKF525, inhibitor of microsomal oxygenase, reduces the mutagenic potential which on the other hand is increased in the presence of 1,1,1-trichloropropene-2-oxide, inhibitor of epoxide hydrolase. This indicates the occurrence of an epoxide intermediate in allyl isothiocyanate metabolism. Another metabolic pathway, namely hydrolysis to allyl alcohol and oxidation to acrolein, a known mutagen, also seems possible as cyanamide, inhibitor of aldehyde dehydrogenase, can slightly increase the mutagenic potential. The reason(s) for allyl isothiocyanate's requirement for long preincubation times to express mutagenicity still requires elucidation, and the question arises: is allyl isothiocyanate a single, exceptional case or not?