The persistence of action, or substantivity, of antimicrobial agents in the mouth appears to be a major variable influencing plaque inhibition. Such substantivity can be assessed by measuring the duration and magnitude of suppression of salivary bacterial numbers produced by antimicrobial agents. Although this has been determined for some agents, there is little information on the substantivity of the numerous products which contain these and other antimicrobial agents. This study was commissioned on the basis that efficacy cannot be assumed merely because a product contains a known active agent. Nine formulations or products were chosen: 2 rinses containing chlorhexidine or C31G, 4 rinses containing cetylpyridinium chloride (CPC) (with and without fluoride and/or alcohol), a minus-CPC control rinse, and 2 toothpastes with and without stannous fluoride. Additionally, water was used as a placebo control. Twenty health dentate volunteers took part in this blind, 10 cell randomized, single rinse, cross-over study, which was balanced for carryover. Mouthrinses were 15 ml volumes and toothpastes 3 gm in 10 ml water slurries rinsed for 60 seconds. On the day of each study volunteers suspended oral hygiene habits and at approximately 9:00 a.m. rinsed with the allocated formulation. Unstimulated saliva samples were obtained immediately before and 30, 60, 180, 300, and 420 minutes after rinsing. The samples were immediately processed for total anaerobic bacterial counts. All rinses except water and the minus CPC control rinse produced significant falls in counts to 30 minutes. Of more relevance in this inter-treatment comparison-designed study, the C31G rinse showed significant substantivity compared to water only for 60 minutes. C31G was highly significantly less substantive than chlorhexidine from 30 minutes to 420 minutes. The CPC rinses were similar and significantly more substantive than their control rinse to between 180 and 300 minutes. The stannous fluoride and control pastes were similarly substantive to 300 minutes, with the stannous fluoride paste remaining substantive compared to water to 430 minutes. Based on antimicrobial action these formulations varied considerably in substantivity and this is likely to reflect their comparative plaque inhibitory properties.