The effectiveness of six commercially available mouthwashes against common buccal organisms was studied. The minimum inhibitory concentrations (MIC) for two of the studied mouthwashes (Corsodyl and Oraldene) against buccal organisms were determined in Todd Hewitt medium with or without 5% serum. The concentration of the active substance in these two mouthwashes was in excess of the corresponding MIC. When the medium was supplemented with serum, lower MIC values were observed. Kill-time determinations, used at half the concentration of the normal preparation, revealed a rapid lethal effect for all tested mouthwashes. The slowest lethal effect was observed with Fluocaril mouthwash. When mouthwashes were tested in volunteers, an immediate significant fall in salivary bacterial counts was produced by all except Fluocaril. With the latter mouthwash the decrease was significant 2-30 minutes after rinsing. The bacterial levels returned to pre-rinse levels after 30 minutes for Listerine, after 90 minutes for both Oraldene and Mint and after 180 minutes for Corsodyl, Fluocaril and Sansilla mouthwashes. The results indicate that there is a good correlation between in vivo efficacy and in vitro determination of all mouthwash preparations.