Plaque was collected from a group of volunteers who used either Listerine antiseptic (LA), its vehicle control (V), or a water control (C) twice daily in addition to their normal toothbrushing in a double-blind controlled clinical experiment. Following the 9-month clinical study, plaque collected from the supragingival surfaces of 20 teeth from each of 78 subjects was weighed wet, freeze dried, reweighed, resuspended, sonicated and estimated for protein. In addition, endotoxin activity was evaluated by means of the limulus lysate assay. A 52.6% reduction in wet weight was found LA versus C (p = 0.04); LA versus V showed a 55.1% reduction (p = 0.03). A 59.0% reduction in dry weight was found LA versus C (p = 0.01); LA versus V showed a 59.6% reduction (p = 0.01). A 59.7% reduction in plaque protein was seen LA versus C (p = 0.01); LA versus V showed a 59.2% decrease (p = 0.02). A 75.8% reduction in limulus lysate activity was found LA versus C (p = 0.01); LA versus V showed a decrease of 77.9% (p = 0.01). Our results demonstrate that LA has a dramatic effect on plaque toxic activity, as measured by a decrease in limulus lysate assay, as well as on its biomass.