A recently reported six-month gingivitis study demonstrated that in subjects with gingivitis, a triclosan/pyrophosphate dentifrice provided supragingival plaque control. The level of plaque reduction was comparable with that reported for other triclosan-containing dentifrices; however, no reductions in gingivitis were observed for triclosan/pyrophosphate relative to the negative control. One possible explanation of this result is that the Hawthorne effect in the study was too great to allow the detection of a treatment benefit for the triclosan product. In order to further explore the relevance of these results, three independent clinical studies were undertaken utilizing designs based on a 21-day experimental gingivitis model in which Hawthorne effects are minimized, in part due to the absence of toothbrushing. In each model, a pre-study prophylaxis was followed by a three-week period of oral hygiene instruction to establish optimum baseline gingival health in study participants. The studies varied in enrollment; 120, 33 and 32 subjects completed treatment on studies 1, 2, and 3, respectively. In study 1, test articles were dentifrice products (0.28% triclosan/5% pyrophosphate/0.145% sodium fluoride, 0.2% triclosan/0.5% zinc citrate/0.112% sodium fluoride, 0.145% sodium fluoride and 0.15% sodium monofluorophosphate) applied neat and undiluted via a performed tooth shield (that prevents mechanical tooth-brushing at the test sites in the oral cavity) in a partial mouth design. In study 2, test articles were also dentifrice products (0.28% triclosan/5% pyrophosphate/0.243% sodium fluoride, 0.3% triclosan/2% Gantrez copolymer/0.24% sodium fluoride and 0.243% sodium fluoride) but administered to subjects in the form of 1:3 aqueous slurry rinses. Lastly, in study 3, test articles were all mouthrinses (0.12% chlorhexidine, 0.045% triclosan in ethanol plus respective vehicle placebos). Clinical assessments to quantify the test articles' effects on the development of plaque and gingivitis were conducted at baseline (studies 1, 2 and 3), day 7 (studies 2 and 3), day 14 (studies 2 and 3) and day 21 (studies 1, 2 and 3). In study 1, no statistically significant treatment effects were observed between the test articles and controls for plaque or gingivitis development. In study 2, no statistically significant treatment effects were observed at any time point between test products for the development of gingivitis. At days 7 and 14, there were no significant differences between test products and control for plaque development as well. At day 21, the group rinsing with the triclosan/pyrophosphate/sodium fluoride slurry had significantly less plaque accumulation than the group rinsing with the triclosan/copolymer/sodium fluoride slurry (p < 0.05); however, neither of the groups using test products containing triclosan was significantly different for plaque development from the group using the sodium fluoride control test article. In addition, aspartate aminotransferase activity in gingival crevicular fluid was assayed at days 0 and 21; no between-group differences were found at either of these time points, though day 21 AST activities were higher than those at baseline. In study 3, statistically significant treatment differences in plaque regrowth and gingivitis were observed at day 21 for the chlorhexidine rinse versus all other rinses (p < 0.05). No other statistically significant treatment effects were observed between test compounds at any other time points. The results benchmark the anti-plaque and anti-gingivitis benefit for a range of triclosan-based product forms against positive and negative controls in a three different experimental gingivitis models, a design considered predictive of clinical efficacy in longer-term investigations. It is concluded that dentifrice products containing triclosan do not possess sufficient antimicrobial activity to suppress plaque and gingivitis development in the absence of normal oral hygiene, and that relative to chlorhexidine, triclosan itself offers only modest efficacy for the prevention of plaque accumulation and therefore the delayed onset of gingivitis.