Sulfur-containing phytochemicals of two different kinds are present in all Brassica oleracea (Cruciferae) vegetables (cabbage, broccoli, etc.). They are glucosinolates (previously called thioglucosides) and S-methyl cysteine sulfoxide. These compounds, which are derived in plant tissue by amino acid biosynthesis, show quite different toxicological effects and appear to possess anticarcinogenic properties. Glucosinolates have been extensively studied since the mid-nineteenth century. They are present in plant foods besides Brassica vegetables with especially high levels in a number of seed meals fed to livestock. About 100 different kinds of glucosinolates are known to exist in the plant kingdom, but only about 10 are present in Brassica. The first toxic effects of isothiocyanates and other hydrolytic products from glucosinolates that were identified were goitre and a general inhibition of iodine uptake by the thyroid. Numerous studies have indicated that the hydrolytic products of at least three glucosinolates, 4-methyl-sulfinylbutyl (glucoraphanin), 2-phenylethyl (gluconasturtiin) and 3-indolylmethyl (glucobrassicin), have anticarcinogenic activity. Indole-3-carbinol, a metabolite of glucobrassicin, has shown inhibitory effects in studies of human breast and ovarian cancers. Kale poisoning, or a severe haemolytic anaemia, was discovered in cattle in Europe in the 1930s, but its link with the hydrolytic product of S-methyl cysteine sulfoxide was only shown about 35 years later. S-methyl cysteine sulfoxide and its metabolite methyl methane thiosulfinate were shown to inhibit chemically-induced genotoxicity in mice. Thus, the cancer chemopreventive effects of Brassica vegetables that have been shown in human and animal studies may be due to the presence of both types of sulfur-containing phytochemicals (i.e. certain glucosinolates and S-methyl cysteine sulfoxide).