Effect of hydrogen peroxide (H2O2) on peripheral blood mononuclear cells (PBMNCs) was determined to assess the antioxidant property of the aqueous extracts of Nigella sativa seeds, Allium sativum, and Rosmarinus officinalis leaves. Lipid peroxidation was measured as marker for oxidative stress. Malondialdehyde significantly increased in the presence of H2O2 indicating extensive lipid peroxidation. The preincubation of PBMNCs with Nigella sativa aqueous extract resulted in a significant reduction of lipid peroxidation in the presence of 10 mM H2O2 at a dose as low as 0.8 mg/ml. However, other two extracts showed a significant decrease in malondialdehyde levels at higher concentrations. Although several studies have reported antioxidant properties of many plant extracts, these studies were investigating essential oils or organic extracts. The present study provides evidence of antioxidant properties in the aqueous extracts of the used plants. The reduction of lipid peroxidation may be attributed to the presence of high levels of water-soluble antioxidant agents.