A double-blind, randomised, 4-week clinical trial with parallel group design in 57 patients with gingivitis was conducted for studying the antibacterial efficacy and safety of a delmopinol HCl aqueous solution 2 mg/ml (0.2% w/v), which was used for unsupervised mouth-rinsing and compared with placebo and chlorhexidine digluconate 2 mg/ml (0.2% w/v, Hibitane Dental, ICI Pharmaceuticals, UK). The plaque index and plaque wet weight were used to measure plaque formation, and gingival fluid flow and bleeding on probing to measure gingivitis. According to the reduction from baseline, chlorhexidine showed a significantly better effect on plaque formation than the placebo after 4 weeks treatment for both plaque measurements. Delmopinol exhibited significantly lower plaque index scores than placebo. The difference between chlorhexidine and delmopinol was not statistically significant for any of the plaque measurements. For gingivitis, no statistically significant differences were obtained between the effects of delmopinol, chlorhexidine and placebo. A transient anaesthetic sensation in the oral mucosa was experienced more clearly by the patients in the delmopinol group than by those using chlorhexidine or placebo rinses. Rinsing with chlorhexidine resulted in more staining of the teeth and tongue than did delmopinol and placebo. The placebo solution tasted better than the 2 active solutions. The results showed that rinsing with either delmopinol HCl aqueous solution 2 mg/ml or chlorhexidine digluconate 2 mg/ml 2x daily for 60 as a supplement to normal oral hygiene, following an initial professional tooth cleaning, leads to a lower plaque formation than rinsing with placebo.(ABSTRACT TRUNCATED AT 250 WORDS)