The aim of this study was to test a possible dose‐response effect of topical application of delmopinol HC1 on the salivary microbiology, the healing of a pre‐established experimental gingivitis, plaque development, and supragingival plaque composition. Forty‐eight healthy subjects were enrolled in an oral hygiene program for 2 wk to upgrade their oral health. After professional tooth cleaning, they abstained from all oral hygiene, but applied 2 ml of a placebo with a soft paintbrush onto their teeth twice daily for 2 wk. At the end of this period, the subjects received tooth cleaning and were then assigned to three treatment groups of 16 individuals each. They applied 2 ml of 0.1%, 0.5%, and 1% delmopinol HC1, respectively, twice daily for the next 2 wk and refrained from all other oral hygiene procedures. At the end of the placebo and delmopinol HC1 treatment periods, (1) saliva samples were obtained and cultivated on a series of media, (2) the degree of gingivitis was measured with gingival crevicular fluid (GCF) and gingivitis index (GI), (3) the stainable buccal plaque extension was analyzed planimetrically, and (4) the bacterial morphotypes of plaque adjacent to the gingival margin were analyzed. No changes in the salivary microbiologic counts were detected. The amounts of GCF and GI were reduced in all delmopinol groups, as compared with placebo. Mean plaque extension was reduced by 16% for the 0.1%, 56% for the 0.5%, and 58% for the 1% delmopinol group. Cocci appeared to predominate in bacterial dental plaque when 0.5% and 1% delmopinol were used. The results indicate that short‐term use of delmopinol promotes the healing of pre‐established gingivitis, reduces plaque formation, and delays plaque maturation without detectable changes in the salivary m