Abstract Cultures of Saccharomyces cerevisiae in minimal medium were inhibited by 1 μg/milliliter or more of methyl-1-(butylcarbamoyl)-2-benzimidazole-carbamate (benomyl). The yeast could reverse the inhibition at concentrations less than 50 μg/milliliter after a time proportional to the concentration of the fungicide, thus indicating the approximate toxicity under the conditions used. Cysteine, and other thiols of the structure R-CH 2-SH tended to reverse the inhibition. This reversal was due to a metabolic effect rather than a direct chemical combination of the thiols with benomyl or methylbenzimidazole carbamate, its breakdown product. Fungitoxic residues were reduced in the medium during yeast growth if benomyl and cysteine were present. However, spectrophotometric comparisons of cultures grown in benomylcysteine mixtures vs. controls using whole washed cells suggested that neither cytochrome P-450 nor any cytochrome normally present were induced. The mechanism of action of the thiols remains to be demonstrated.