A number of chlorhexidine mouthwashes are available commerically which differ in formulation and regimen of use. As a comparative measure of antimicrobial persistance, this study evaluated the effect of 4 cylorhexidine mouthwash formulations on salivary bacterial counts after a single rinse. The study was a randomised single-examiner blind 5-way crossover investigration employing a panel of 10 young healty human volunteers. The 0.12% and 0.2% commerical commerical formulations when rinsed according to the respective manufacturers instructions produced similar large and prolonged reductions in salivary bacterial counts during the 7-h period. A 0.1% formulation also commercially available produced minimal effects of salivary bacterial counts and was little different to the saline rinse. A reformulated 0.1% rinse, not commerically availble at the time of the study, produced significant salivary bacterial count reductions over the 7-h period albeit to a lesser degree of magnitude than to the 0.2% and 0.12% rinses. The results were consistent with comparative plaque inhibitory studies of the formulations and suggest that the method is a quick and simple way of screening products for antimicrobial and antiplaque potential.