Selenium (Se), an essential micronutrient whose absence causes skeletal and cardiac muscle dysfunction (Young, 1981; Sathe et al., 1992), is required for the proper function of the immune system and for cellular defense against oxidative damage and thus may play a role in the prevention of cancer and premature aging (Axley et al., 1991). Selenocysteine (1, HSeCH₂CH(NH₂)-COOH; Cys-SeH), whose incorporation into proteins is directed by a UGA codon and which has been called the 21st amino acid essential for ribosome-directed protein synthesis (Soll, 1988), is present at the active sites of glutathione peroxidase, 5'-deiodinase and selenoprotein P (Burk and Hill, 1994). Because animal Se deficiencies are typically treated by dietary supplementation with sulfur-containing amino acids and because Allium and Brassica spp. are rich sources of such amino acids, we sought to determine whether 1 and related selenoamino acids are also found in garlic (Allium sativum) and broccoli (Brassica oleracea botrytis). Garlic is claimed to contain selenoproteins (Wang et al., 1989) and a selenopolysaccharide (Yang et al., 1992), while broccoli is said to accumulate high levels of unknown forms of Se (Bañuelos et al., 1993). Cabbage (Brassica oleracea capitata) grown with H₂SeO₃ contained various selenoamino acids, -peptides, and -proteins (Hamilton, 1975). Development of a simple analytical procedure for selenoamino acid determination would also be useful in the examination of plants grown in high soil Se areas, e.g., regions of central California (Bañuelos et al., 1993). Since the natural S:Se ratio in garlic is ca. 1.2 x 10⁴:1 and since the properties of S- and Se-containing amino acids are similar, leading to coelution problems (Tschursin et al., 1994), we employed the highly sensitive, element-specific technique of gas chromatography with atomic emission detection (GC-AED; Cai et al., 1994a,b) in the analysis of ClCO₂Et-derivatized amino acids. We have identified for the first time Se-containing amino acids in unenriched as well as Se-enriched garlic and broccoli and have also identified selenoamino acids in Se-enriched onion, obtaining different results than previously reported (Spåre and Virtanen, 1964). In the accompanying paper we identify organoselenium compounds in human breath following consumption of fresh garlic (Cai et al., 1995), suggesting that they are derived from degradation of the naturally present selenoamino acids. Finally, we propose that reduction in incidence of mouse breast tumors in carcinogen-treated mice fed Se-enriched garlic (Ip et al., 1992) is due to the enhanced levels of 1 and Se-methyl selenocysteine (2; Cys-Se-Me) in these plants.