Group B streptococci (GBS) lack catalase, and they produce and release H2O2;thus, they should be readily killed by phagocytes with a diminished respiratory burst. Surprisingly, although strains of Staphylococcus aureus were killed at H2O2 concentrations greater than 0.5 mM, GBS strains were killed only at concentrations greater than 5mM. In contrast, GBS were killed by hydroxyl radicals generated by the xanthine oxidase-acetaldehyde system at O2 fluxes greater than or equal to 3.5 nmol/ml per min, whereas O2 fluxes greater than or equal to 10 nmol/ml per min were required to kill the S. aureus strains. Results with virulent and laboratory strains of GBS were similar. The differences in susceptibility of GBS and S. aureus seemed to correlate with differences in content of endogenous oxygen-metabolite scavengers. GBS contained approximately 100-fold more glutathione and approximately 20-fold more glutathione reductase than did S. aureus, whereas S. aureus was rich in catalase that GBS lacked. GBS that were grown in buthionine sulfoximine, however, contained 87% less glutathione than did controls but were not more susceptible to killing by H2O2 or the xanthine oxidase-acetaldehyde system. Similarly, the relative susceptibility of GBS to tert-butyl hydroperoxide and H2O2 paralleled that of S. aureus. Thus, inherent differences in susceptibility of vital cellular functions to oxidative damage rather than content of oxygen-metabolite scavengers may account for the differences in susceptibility of GBS and S. aureus.