SUMMARY: The inhibition by triazole of the growth of wild-type Salmonella typhimurium is reversed by serine, methionine or cysteine. Two auxotrophs which responded to any one of these three compounds were hypersensitive to triazole and were shown by genetical analysis and enzyme assays to have a cysE (serine transacetylase) deficiency. Triazole hypersensitivity of the mutants was reversed by sulphite and sulphide and triazole prevented induction of sulphate permease and activating enzymes by O-acetyl serine (OAS) or serine in the wild-type. It is probable that the inhibitory effect of triazole is due to this interference with the induction of cysteine biosynthetic enzymes.