The physiological relevance of a novel thiol methyltransferase from cabbage, and its possible role in sulphur metabolism have been investigated. The enzyme was absent from the chloroplast, the site of sulphate reduction, and was localized in the cytosol. Potential substrates were initially screened on the basis of their ability to inhibit the methylation of iodide, a previously known substrate for the enzyme. Thiocyanate, 4,4'-thiobisbenzenethiol, thiophenol, and thiosalicylic acid were identified as possible substrates. Methylation of these thiols by the purified enzyme using [Methyl-3H]S-adenosyl-L-methionine confirmed their nature as substrates. The purified enzyme strongly preferred thiocyanate as a methyl acceptor. The enzyme had K(m) values of 11, 51, 250 and 746 mmol m-3 for thiocyanate, 4,4'-thiobisbenzenethiol, thiophenol and thiosalicylic acid, respectively. The identity of methylthiocyanate as the product of thiocyanate methylation by the purified enzyme was confirmed by mass spectrometry. The enzyme was strictly associated with glucosinolate-containing plants. Thiol substrates of the enzyme are known products of glucosinolate hydrolysis. Our observations indicate that this enzyme could be involved in the detoxification of reactive thiols produced upon glucosinolate degradation in these plants.