Syntheses are reported for gamma-glutamyl Se-methylselenocysteine (Sa), selenolanthionine (16), Se-1-propenylselenocysteine (Gd), Se-2-methyl-2-propenyl-L-selenocysteine (6e), and Se-2-propynyl-L-selenocysteine (6f). Oxidation of 8a and Se-methylselenocysteine (Ga) gives methaneseleninic acid (24), characterized by X-ray crystallography, and dimethyl diselenide (25). Oxidation of Se-2-propenyl-L-selenocysteine (6c) gives allyl alcohol and 3-seleninoalanine (22). Compound 22 is also formed on oxidation of 16 and selenocystine (4). Oxidation of 6d gives 2-[(E,Z)-1-propenylseleno]propanal (36). These oxidations occur by way of selenoxides, detected by chromatographic and spectroscopic methods. The natural occurrence of many of the Se-alk(en)ylselenocysteines and their gamma-glutamyl derivatives and oxidation products is discussed. Three homologues of the potent cancer chemoprevention agents 6a and 6c, namely 6d-f, were evaluated for effects on cell growth, induction of apoptosis, and DNA-damaging activity using two murine mammary epithelial cell lines. Although each compound displays a unique profile of activity, none of these compounds (Gd-f) is likely to exceed the chemopreventive efficacy of selenocysteine Se-conjugates Ga and 6c.