Historically, the fruit of Tribulus terrestris Linn. (Zygophyllaceae) has been used in India and China as a constituent of rejuvenation tonics. It is also used in traditional Indian medicine in the therapy of a variety of health conditions affecting liver, kidney, and cardiovascular and immune systems. Oxidative stress has been implicated in some of these disease conditions. Therefore, we have investigated the antioxidant activity of the aqueous extract of Tribulus terrestris fruit (TTE) in spleen cells. Generation of intracellular reactive oxygen species (ROS) in response to γ-radiation and 2,2'-azobis(2-propionimidinedihydrochloride) (AAPH) was measured by flow cytometry using dichlorodihydrofluorescein diacetate (H₂DCFDA). TTE scavenged ROS induced by γ-radiation as well as AAPH in a concentration-dependent manner. TTE also showed protection against oxidative stress-induced apoptosis. In addition, TTE exhibited mitogenic activity in the spleen cells. We conclude that TTE attenuates oxidative stress and protects cells from lethal oxidant damage, and thus its therapeutic potential in treating many ailments may relate to its antioxidant properties.