In this study, the new stable sulfur-containing compounds onionins A₂ ( 1 ) and A₃ ( 2 ) were isolated from the acetone extracts of the bulbs of Allium cepa L. and identified as the stereoisomers of onionin A₁ discovered in our previous study. Their chemical structures, 3,4-dimethyl-5-(1 E -propenyl)-tetrahydrothiophene-2-sulfenic acid- S -oxides, were characterized using various spectroscopic techniques. In addition, 1 and 2 together with onionin A₁ were successfully isolated from the leaves of the Welsh onion, Allium fistulosum L. The onion-extracted fractions showed good potential to inhibit the polarization of M2 activated macrophages, indicating their possible ability to inhibit tumor cell proliferation.