Biofilms of Streptococcus sanguis and planktonic cells of the organism were exposed to chlorhexidine gluconate and cetylpyridinium chloride, at concentrations used clinically, and survivors enumerated. S. sanguis exhibited a lower susceptibility to both antiseptics when it comprised a biofilm than when the organism was in the planktonic form. No viable bacteria were detectable after 5 min of exposure of planktonic cells to 0.2% (w/v) chlorhexidine gluconate or 0.05% (w/v) cetylpyridinium chloride, whereas viable bacterial survived in biofilms of S. sanguis even after exposure to these agents for 4 h. Older biofilms (7 days old) had similar susceptibilities to the antiseptics as younger biofilms (4 days old). Chlorhexidine achieved kills corresponding to approximately a 2 log10 reduction in the viable count of biofilms, containing approximately 10(7) colony-forming units after 5 min of incubation, whereas the corresponding kills achieved by cetylpyridinium chloride amounted to approximately a 1 log10 reduction. However, on a molar basis, cetylpyridinium chloride was the more effective of the two antiseptics. Minimum inhibitory concentration determinations showed chlorhexidine gluconate to be more effective against S. sanguis than cetylpyridinium chloride. The results of this study have revealed that the minimum inhibitory concentration is not a reliable predictor of the relative effectiveness of antimicrobial agents against S. sanguis biofilms.