Reactive oxygen species (ROS) are generated when oxygen is supplied in excess and/or its reduction is insufficient. Despite of their toxicrty towards microorganisms, ROS are implicated in a variety of pathological processes and diseases such as rheumatic and pulmonary chronic inflammations, ischaemid reperfusion states, and aging (1,2,3). ROS, generated from activated inflammatory cells due to the respiratory burst process, exert damaging effect to the cell components by oxidation of macromolecules (lipids, proteins, DNA) (4,5).Flavonoids are phenolic compounds, phenyl-2-chromone derivatives. Their basic nucleus contains fmeen carbon atoms arranged in C6-C3-C6 configuration, that is two aromatic rings, A and B, linked by a three corbon heterocyclic ring C (figure 1). Flavonoids are widely distributed in plant kingdom and known to possess a variety of biological and pharmacological properties in vivo and in Mro, such as anti-allergic and anti-inflammatory activities (6,7). Over the last few years, several derivatives of flavonoids, were shown to exhibit antioxidant activity (8,9,10,11). Nakayama et a/. (12) reported that either the dihydroxy structure in the B ring or certain structures in the A and C rings of the flavonoids are necessary for the protection against cytotoxicity of active oxygen species. Furthermore, Kim et a/.(13) reported that C-2,3-double bond is necessary for in vivo anti-inflammatory actiwty of flavonoids and the potencies of this activity depend on the patterns and numbers of hydroxylation on AIB ring.Cleome species are plants abundantly distributed in the North of Africa. The plant leaves are well reported in the indegenous system of medicin to have an almost immediate effect on relieving abdominal and rheumatic pains (1 4). In a previous work (15), we have observed that the extract of C / m arabica leaves possesses an anti-inflammatory activty in rat model.As part of pharmacological investigation, we now report the in vifro antioxidant activity of the ethyl acetate crude extract of Cleome arabica. Fresh leaves were collected from Bossaada, South-East of Algeria, and dried in dark at room temperature. A yellow crude precipitate was obtained using ethanol/ H2O and ethyl acetate successively. Both thin layer chromatography (CHCD: MeOH, 7:3) and UV analysis showed that the crude extract contains a mixture of mainly two flavonols. The extract was tested for its antioxidant actity towards oxygen reactive species generation: superoxide anion (0; ) hydrogen peroxide (H202) and hypochlorous acid (HOCI ). These three oxygen reactive species were generated in human leucocyte-polymorphonuclear Neutrophils L-PMN (2.5 x 1 O6 Iml) stimulated by Phorbol 13-Myristate 12-Acetate (PMA) at a concentration of 160 nM. L-PMN, obtained from healthy adults bloob by densty gradient on Histopaque-1077, were incubated in Hanks' HEPES medium pH 7.4 at 37 "C with or without 1OOpl of the supernatant of extract at a concentration of 10 mg/ml ir! culture medium. The amount of superoxide anion was determined following the ...