The authors have succeeded in isolating alliin in a pure, crystalline form and in defining its properties. Alliin is characteristic for certain kinds of garlic; it contains sulphur and nitrogen and represents the initial substance of garlic oil. Alliin itself has no bactericidal action. Upon decomposition—caused by a specific concomitant enzyme, alliinase—the highly bactericidal substance, allicin, is produced. Further decomposition yields the volatile, sharply-odorous allyl-sulphides.