The hydrogen peroxide scavenger catalase prevents the cytotoxic effect of ascorbic acid-mediated hydrogen peroxide production. The addition of exogenous catalase to cells of the U-251 cell line incubated with toxic concentrations of ascorbic acid (10 mmol/L) prevented the lethal effect elicited by ascorbic acid. U-251 cells incubated with catalase alone were not affected in their viability. The results shown are representative for 3 independent analyses and values are expressed as mean ± standard deviation of hexaplicates. The difference between 0 and 250 units/mL catalase is significant (p = 0.004). N.s.: not significant.