The anticaries action of topically applied sodium fluoride (NaF) preparations has been attributed, at least in part, to the antibacterial effect of fluoride on cariogenic bacteria. In this report we examined the bactericidal activity of acidulated NaF gels against preformed plaque aggregates of Streptococcus mutans 6715. Both commercially available and laboratory-prepared gels were tested. Prepared NaF gels were acidified to various pH levels with HCl, H3PO4, or HF. The aggregates of S. mutans were immersed in the various fluoride gels for 5 min and then monitored for viability. None of the commercially available NaF gels with pHs of 3.5 to 4.5 and fluoride concentrations of 5,000 to 12,300 micrograms/ml were capable of exerting complete bactericidal activity against aggregates of S. mutans. For the laboratory-prepared NaF gels, neither the source of hydrogen ion nor the source of fluoride influenced the outcome when fluoride was present at the appropriate MBC and pH. NaF gels with a fluoride concentration of 37,000 micrograms/ml were bactericidal to S. mutans at pH 3.0. At pH 2.5 and 2.0, approximately 12,000 and 1,100 micrograms of fluoride per ml, respectively, was bactericidal. Results from this study suggest that the bactericidal properties of acidulated NaF gels can be enhanced by lowering their pHs to below 3.0.