The synthetic selenium compound methylseleninic acid (MSA) is a direct precursor of active methylselenol and appears to be the best candidate for studies on the mechanisms of selenium cancer prevention and therapy in vitro. Reduced glutathione (GSH) is critical to MSA metabolism, in addition to being a protective antioxidant which scavenges reactive oxygen species (ROS) and maintains the stability of intracellular redox status. In this study, we demonstrated that MSA has an anticancer effect in the human lung cancer cell lines L9981 and 95D using growth inhibition detection, cell-cycle analysis, and apoptosis detection. We examined the role of intracellular GSH content and detected the ROS induced by MSA by fluorescence microscopy, and we used flow cytometry to quantify the ROS induced by pretreatment and co-treatment with N-acetylcysteine (NAC) and MSA. We also confirmed oxidative stress in MSA-induced apoptosis. MSA inhibited lung cancer cell lines L9981 and 95-D growth significantly, induced cell-cycle arrest in the G1 phase and induced apoptosis. Compared to the control group, MSA significantly decreased intracellular GSH content in L9981 cells at higher concentrations of MSA (5 and 7.5 µM), while the intracellular GSH level was also dramatically decreased in L9981 cells treated with 5 µM MSA at different time points of 12- and 24-h (decreased to about 50% and 20% of the control, respectively). Pretreatment with either NAC (GSH synthesis precursor) or buthionine sulfoximine (BSO, GSH synthesis inhibitor) in L9981 cells significantly inhibited the anti-proliferative effect of MSA. MSA induced the generation of ROS, which was significantly reduced by NAC pretreatment. Furthermore, we also confirmed these results in another lung cancer cell line 95-D. These results suggest that generation of ROS may be essential for the induction of oxidative stress and apoptosis by MSA in L9981 and 95-D lung cancer cells. The balance between oxidative stress induced by MSA and the antioxidant effect exerted by intracellular GSH content may determine the ultimate outcome after MSA treatment. Copyright © 2011 Wiley Periodicals, Inc.