This double-blind, randomised, 6-month clinical trial with parallel group design in 68 subjects with gingivitis was conducted to study the effects on the oral flora of delmopinol hydrochloride 2 mg/ml (0.2% w/v, Decapinol Mouthwash), when used for partly supervised mouthrinsing in comparison with placebo and chlorhexidine digluconate 2 mg/ml (0.2% w/v, Hibitane Dental, ICI Pharmaceuticals, UK). Apart from estimating the total cultivable microbial dental plaque flora and salivary flora, analyses were focused on bacterial groups associated with gingivitis/periodontitis and dental caries. Furthermore, the presence of staphylococci, gram-negative enteric bacteria and yeasts in saliva were evaluated. The minimal inhibitory concentration (MIC) was determined for isolates belonging to the predominating micro-organisms in samples of both dental plaque and saliva. In relation to the findings in the placebo group, the use of delmopinol during the rinsing period did not produce an undesirable shift in the bacterial populations considered to be related to dental caries or periodontal diseases. These groups remained virtually unchanged during the study. In relation to the observations in the placebo group, slight reductions in the total cultivable plaque and salivary flora were observed during the study and no change was found in the ratio total anaerobically/aerobically cultivable microbial flora. Furthermore, no increased growth in staphylococci, enteric bacteria or yeasts was observed in the saliva samples. The pattern of changes taking place in the composition of the plaque and salivary microbial flora in samples from the participants rinsing with chlorhexidine were in most aspects similar to that observed in the delmopinol group. In the delmopinol group, no microbiologically significant changes were observed over time in the MIC-values for the isolates, neither in the plaque nor in the saliva samples, which indicates that no adaptation to delmopinol had taken place during the rinsing period. Similar observations were made for the plaque isolates in samples from the participants in the chlorhexidine group. On the other hand, when gram-positive and catalase-negative cocci from the saliva samples of the latter group were tested against chlorhexidine, 4-6 times higher MIC-values were obtained at 3 and 6 months both when compared to baseline and in comparison with the other two rinsing groups (p<0.01 or p<0.05). Neither delmopinol nor chlorhexidine showed any residual effect on the studied microbial groups in the plaque and the saliva samples 3 months after the end of treatment. In conclusion, delmopinol was accompanied by a composition of the plaque and salivary flora associated with healthy conditions in the oral cavity.