The antibacterial activity and adherence-enhancing effects of nonoxynol-9 were evaluated against vaginal and uropathogenic bacteria. Nonoxynol-9 was markedly less active against the 43 uropathogenic bacterial and yeast strains tested (MIC90, greater than 32%) than against the 26 Gardnerella vaginalis strains (MIC90, less than or equal to 0.015%) and the 53 Lactobacillus strains (MIC90, 8%) tested. Hydrogen peroxide-producing strains of Lactobacillus were more susceptible to nonoxynol-9 (MIC90, 4%) than nonproducers (MIC90, 16%). Two Escherichia coli strains that expressed type 1 fimbriae and three vaginal strains of lactobacilli adhered in significantly higher numbers to vaginal epithelial cells preincubated with 5% nonoxynol-9 than to control cells preincubated with PBS. Spermicides may provide a selective advantage in colonizing the vagina with nonoxynol-9-resistant uropathogens such as E. coli, perhaps via a reduction in vaginal lactobacilli (especially hydrogen peroxide-producing strains) and through enhancement of adherence of E. coli to epithelial cells.