Abstract: The changes that take place in the leaf apoplastic antioxidant enzymes system, protein and isoenzyme pattern in response to salts stress was studied in three onion (Allium ceba L.) cultivars, named Behary Red, Giza 6 and Giza 20. The constitutive level of lipid peroxidation and glutathione content (GSH), enzymes activity of catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) as well as the protein and isoenzymes pattern were performed in the leaf of control (tap water, 200 ppm salts) and three concentrations of salt (2000, 4000 and 6000 ppm salts prepared from seawater (28000 ppm salts) stressed plants, 45 days after completing the stress treatments. The salt stress increased the rates of lipid peroxidation (as indicated by increasing the malonaldehyde (MDA) contents) and their effect was increased gradually by increasing salts levels. The increasing values in MDA contents in salt stress plants treated at 2000, 4000 and 6000 ppm salts levels were 121.9, 248.6 and 290.3% in Behary Red cv. and 126.4, 260.0 and 396.2% in Giza 20 cv., respectively of the control level (100%). Generally, salts stress induced significant increase in the activities of the antioxidative enzymes CAT, POD and SOD in onion cvs., accompanied by increased in glutathione level. The GSH level was increased more in Behary Red cv. and Giza 20 cv., than in Giza 6 cv., at 6000 ppm salts. Their increasing values were 203.7, 283.3 and 242.86%, respectively compared to the control level (100%). The leaf protein profile obtained by SDS-PAGE was diagnostic of characteristics different onion cultivars exposed to salts stress which, three distinct new protein bands with molecular weigh 50, 66 and 80 kDa were produced in Giza 6, Giza 20 and Behary Red onion cultivars, respectively. Also, electrophoretic bands of POD and CAT isoenzyme of different onion cultiver were differ in numbers and relative concentration due to salts stress. This finding suggested that Behary onion cultiver is better protected against lipid peroxidation process, inherently and under salts stress, than Giza 6 and Giza 20 onion cultivers. Also, the new protein bands and the characteristic CAT and POD isoenzymes banding patterns could be used as a biochemical marker for selection of salt tolerance onion plants when grown under salts stress.