1. Both after ingestion of benzyl isothiocyanate (BITC), a compound with antibacterial properties, and after consumption of garden cress known to contain BITC, the metabolite N-acetyl-S-(N-benzylthiocarbamoyl)-L-cysteine was identified in the urine of volunteers by comparative chromatography. 2. The chemical structure of the metabolite was confirmed by elemental analysis and by comparison of the i.r. and 1H-n.m.r. spectra with those of the synthetic product. 3. On average 53.7% of the dose of BITC was excreted as this metabolite by the renal route. 4. The metabolite was excreted rapidly, appearing with maximum concentrations some 2-6 h after dosing and being essentially complete 10-12 h after administration.