The efficacy of 10 mouthwashes available in Bangkok in inhibiting Streptococcus mutans growth in vitro was studied. After one-minute incubation with the bacterium, most mouthwashes, at concentrations recommended by the manufacturers, demonstrated the antimicrobial effect; three exhibited a complete while five a partial but varying degree of inhibition. Only two mouthwashes were found that did not significantly show any detrimental effect on the bacterial viability.