BspA is an abundant surface protein from Lactobacillus fermentum BR11, and is required for normal cystine uptake. In previous studies, a mutant strain deficient in BspA (L. fermentum PNG201) was found to be sensitive to oxidative stress. In this study, the biochemical basis for this was explored. It was found that under aerobic batch culture conditions in de Mann-Rogosa-Sharpe medium, both L. fermentum BR11 and PNG201 entered stationary phase due to hydrogen peroxide accumulation. However, this took place at a lower optical density for PNG201 than for BR11. Measurements of hydrogen peroxide levels revealed that the BspA mutant strain overproduces this compound. Addition of 6 mM cystine to aerobic cultures was found to prevent hydrogen peroxide production by both the BR11 and PNG201 strains, but lower cystine concentrations depressed hydrogen peroxide production in BR11 more efficiently than in PNG201. Each mole of cystine was able to prevent the production of several moles of hydrogen peroxide by L. fermentum BR11, suggesting that hydrogen peroxide breakdown is dependent upon a thiol that cycles between reduced and oxidized states. It was concluded that peroxide breakdown by L. fermentum BR11 is dependent upon exogenous cystine. It is most probable that the imported L-cystine is catabolized by a cystathionine lyase and then converted into a thiol reductant for a peroxidase.