Onion and garlic contain many sulfur containing active principles mainly in the form of cysteine derivatives, viz. S-alkyl cysteine sulfoxides which decompose into a variety of thiosulfinates and polysulfides by the action of an enzyme allinase on extraction. Decomposed products are volatile and present in the oils of onion and garlic. They possess antidiabetic, antibiotic, hypocholesterolaemic, fibrinolytic and various other biological actions. In addition to free sulfoxides in alliums, there are nonvolatile sulfur-containing peptides and proteins which possess various activities and thus make these vegetables as an important source of therapeutic agents. As allyl and related sulfoxides are inhibiting thiol group enzymes, alliums are to be used only in limited quantities.