Isolation and identification of (S(S)R(C))-S-n-butylcysteine sulfoxide (1) from the bulbs of Allium siculum is reported. This compound was found in all parts of the plant (bulbs, stem, leaves, and flowers) along with known compounds (S(S)R(C))-S-methyl- and (R(S)R(C))-(E)-S-(1-propenyl)cysteine sulfoxides (2 and 3, respectively). In addition, six n-butyl-containing thiosulfinates (4-9) have been found in a CH2Cl2 extract of the bulbs. Structures were determined by a combination of spectral methods (primarily NMR and MS) and by comparison with authentic compounds obtained by synthesis. Antimicrobial activities of 4-7 and 9 are reported.