The aim of the study was to investigate the effect of delmopinol hydrochloride on the development of dental plaque and on newly established plaque. In addition, the influence of this compound on the composition of the microbiota colonizing the gingival mucous membrane was studied. 14 healthy male volunteers took part. After a 3 week pre-experimental period of intense oral hygiene, the participants refrained from all oral hygiene for 14 days. The buccal surfaces of cuspids and bicuspids on one side of the jaws were treated with a 1% aqueous solution of delmopinol hydrochloride (applied with a paint brush) 2 x a day for 7 days, while the contralateral side received placebo solution. On day 7, the application procedures were changed in that the test compound was applied on the teeth previously treated with placebo and vice versa. Plaque development was assessed clinically and by photo-based planimetric determination. The clinical recordings revealed that 89.3% of the placebo-treated surfaces displayed visible plaque on day 7, compared to 6.0% of the delmopinol hydrochloride treated surfaces. Delmopinol hydrochloride treatment of the previously placebo-treated surfaces resulted in a decrease in the number of surfaces with visible plaque from 89.3% on day 7 to 6% on day 14. These results were confirmed by the planimetric data. No significant change in the composition of the mucosal flora was observed during the experimental period. The present results indicate that delmopinol hydrochloride markedly reduces the formation of dental plaque on a clean tooth surface exposed to conditions which favour bacterial colonization. Furthermore, the substance appears to possess plaque-dissolving properties.