Biofilm formation has been demonstrated as a potentially important mechanism contributing to antibiotic treatment failure on Streptococcus pyogenes. It could play a significant role in recurrent and chronic infections. Boesenbergia pandurata (Roxb.) Schltr., Eleutherine americana Merr. and Rhodomyrtus tomentosa (Aiton) Hassk. have been previously reported from our laboratory as effective agents against S. pyogenes. Therefore, in the present study, we observed the effect of these plants on biofilm formation. The bacterial biofilms were quantified by safranin staining and absorbance at 492 nm. The results clearly demonstrated that all subinhibitory concentrations [1/32-1/2 minimal inhibitory concentration (MIC)] of E. americana (7.81-125 microg mL(-1)) and R. tomentosa (0.24-7.81 microg mL(-1)) extracts significantly prevented biofilm formation while 1/2MIC (7.81 microg mL(-1)) of B. pandurata extract produced this effect. The issue of antiquorum sensing of this pathogenic bacterium has been further explored. A correlation between antiquorum-sensing and antibiofilm-producing activities was demonstrated. Strong inhibition on quorum sensing was displayed with the extract of R. tomentosa. Eleutherine americana extract showed partial inhibition, while B. pandurata did not show this activity. By contrast, an assay of microbial adhesion to hydrocarbon revealed no changes in the cell-surface hydrophobicity of the treated organisms. Active organisms with the ability to inhibit quorum sensing and biofilm formation are worth studying as they may provide complimentary medicine for biofilm-associated infections.