A mixed-culture chemostat system has been used as a more stringent laboratory system for evaluation of the antimicrobial effects of Triclosan and zinc citrate. The inhibitors were added alone, and in combination, as a pulse (a high initial inhibitor concentration which decreased with time) or as a dose (concentration of inhibitor increased with time) to give maximum concentrations of 34.5 μmol/L Triclosan and 39.8 μmol/L zinc citrate. When dosed, Triclosan inhibited A. viscosus and all five Gram-negative species, whereas zinc citrate had less effect, probably due to complexation by media components. Similar effects were seen when Triclosan was pulsed, except that S. mutans was the most sensitive Gram-positive species and V. dispar was unaffected. However, when the inhibitors were dosed or pulsed in combination, marked complementary and additive inhibitory effects were observed, particularly against Gram-negative species, although S. gordonii and S. oralis were relatively unaffected. The data confirm that increased effects can be obtained with suitable combinations of antimicrobial agents and suggest that, under certain conditions, apparently broad-spectrum antimicrobial agents may be acting more selectively than hitherto suspected.