Phytochemicals, nutraceuticals and herbal products, such as silymarin, have gained ever growing popularity and are valued for their ability to defend against almost all types of disease. Primarily known as hepatoprotective and antioxidant, silymarin has antimitotic effects against many different cancer models. We intended to determine whether silymarin protects against H2O2-induced toxicity and affects survival rate of rat glioma (C6) and primary glial cells. The role of 1, 10, 50, 75 and 100 mu M silymarin concentrations were tested for 3.5, 24 or 48 h on C6, and primary glia cells that were obtained from 1-3 day old rat brains. Combined with these silymarin doses, 100 mu M H2O2 was applied to the cultures for 3 h, then the medium was removed and re-fed for 24 h. The percentage of surviving cells was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, thyazolyl blue method. All silymarin doses could not show any clear influence against H2O2 toxicity on both cell types; however, treatment of both cells with respective doses of silymarin for 3.5, 24 or 48 h reduced the cell number ranging from 13 to 73%. IC50 values of silymarin were calculated to be 50 mu M for C6 and 79 mu M for primary glia after 24 h. After 48 h, IC50 values were 43 mu M for C6 and 68 mu M for primary glia cells. Although silymarin dose and time dependence was more effective on decreasing glioma survival, it also reduced significantly primary glia survival.