Methods developed for the separation of S-alk(en)yl- L-cysteines and their corresponding (+/-)-sulfoxide isomers by reversed-phase HPLC were applied to the analysis of various garlic samples including fresh garlic, dried extracts, and garlic preparations. Extracts were chromatographed following extraction with 50:50 methanol/water, sample clean-up using Bond Elut C18 or SCX cartridges, and pre-column derivatization with O-phthaldialdehyde/ TERT.-butylthiol.(+)- S-Methyl- L-cysteine sulfoxide and (+)- S-allyl- L-cysteine sulfoxide (alliin) were the only compounds which were identified with certainty. Other sulfur amino acids reported to occur in garlic were absent or were below detection limits under standard chromatographic conditions. Assays for alliin, which is an antibiotic precursor and may be used for standardization of garlic preparations, resulted in great variation between samples: alliin contents were found to range from < 0.1% to 1.15% fresh weight. The accuracy and precision of the assay method, including external calibration of alliin, were evaluated.