Lactobacilli from women with and without bacterial vaginosis (BV) were tested for H2O2 production. Thirty-seven (79%) of the 47 strains of lactobacilli isolated from the women without BV produced H2O2, while only nine (23%) of the 39 strains of lactobacilli obtained from women with BV did so. Five of 20 H2O2-producing and two of 26 non-producing strains of Lactobacillus exhibited antibiosis against four of 12 strains of peptostreptococci and two of 10 strains of Mobiluncus spp. None of a further 41 different anaerobic and facultative anaerobic bacterial strains were inhibited by any of the isolates of lactobacilli tested. Some strains of Gardnerella vaginalis, Bacteriodes spp., Mobiluncus spp. and Peptostreptococcus spp. inhibited the growth of three strains of lactobacilli belonging to different species. When the pH of the culture medium was increased from 6.0 to 6.5 this led to a decrease in the number of strains inhibited and/or the size of the growth-inhibitory zones. Different concentrations of H2O2 did not inhibit any of the strains tested. The growth-inhibitory effect of lactobacilli could not be related to their bacteriocin production. Increasing the iron content of the medium by adding FeCl3 (0.01 mM-1 mM) decreased or completely abolished the antibiosis.