Objective: We examined the antimicrobial activity and composition of vaginal fluid. Study Design: Vaginal fluid from preweighed tampons was assayed for pH, lactic acid, and antimicrobial polypeptides. The fluid was also fractionated by molecular filtration. Antimicrobial activity of whole fluid was determined against representative resident and exogenous microbes, and its fractions were tested against Escherichia coli.Results: Vaginal fluids (5/5 donors) were permissive for Lactobacillus crispatus and vaginalis and Candida albicans, but not for Escherichia coli, Streptococcus group B, and Lactobacillus jensenii in three of five donors. The antimicrobial activity against E coli was predominantly in a <3-kd fraction and correlated with both low pH and high lactic acid content. Compared with a matched pH buffer, lactic acid markedly suppressed the growth of E coli. Concentrated 2- or 5-fold, the protein-rich fraction was active against E coli.Conclusion: Vaginal fluid exerts selective antimicrobial activity against nonresident bacterial species. The activity is mediated by lactic acid, low pH, and antimicrobial polypeptides. (Am J Obstet Gynecol 2002;187:561-8.)