H2O2-generating lactobacilli (LB+) are present in the vagina of most normal women but are absent from most women with bacterial vaginosis (BV). LB+ at high concentration was toxic to Gardnerella vaginalis (the predominant organism in the vagina of women with BV); when the LB+ was lowered to a level where it was ineffective alone, the addition of myeloperoxidase and chloride reinstituted toxicity. Toxicity was inhibited by catalase and was not seen when H2O2-negative lactobacilli were used, implicating H2O2 as the toxic molecule. LB+ could be replaced by H2O2 and chloride by iodide, bromide, or thiocyanate. The optimum pH for inhibition of G. vaginalis was 5.0-6.0 LB+ also was autoinhibitory when combined with myeloperoxidase and chloride. LB+ alone at low concentrations was toxic to Bacteroides bivius through the formation of H2O2. Adequate amounts of peroxidase were found in the vagina of 17 of 21 women. These findings suggest that LB+ may contribute to the control of the vaginal flora, particularly in the presence of peroxidase and a halide.