Recent epidemiological studies proposed that glutathione S-transferase (GST) Ti null genotype was correlated with an increased susceptibility to diseases associated with oxidative stress, including cancer. A comparative study using erythrocytes from individuals with GSTT1 null genotype was carried out to determine how resistance to oxidative stress is affected by lack of this gene, and whether the CST status of a person is an important factor in risk toward oxidant chemicals. Malondialdehyde and carbonyl levels and fluorescence and chemiluminescence formation were used as biomarkers of oxidative stress in erythrocytes exposed in vitro to cumene hydroperoxide (CumOOH), an oxidizing agent. When peroxidation-dependent changes in these parameters were compared between GSTT1 null genotype and controls, who are both GSTM1 and GSTT1 positive, no significant differences were found between the two genotypes, although the erythrocytes of the GSTT1 null group had lower GSTT1 activity toward CumOOH. Our results indicate that erythrocytes from individuals with GSTT1 null genotype are not abnormally susceptible to CumOOH-induced oxidant challenge.