Abstract. The purpose of the present study was to establish in vitro the inhibiting effect of a herbal extract mixture on a selected number of micro-organisms and to test in vivo the effect of a mouthwash containing 6.3 mg/ml herbal extract mixture on plaque and gingivitis as compared to a minus active control mouthrinse. The herbal extract was a mixture of: Juniperus communis (juniper), Urtica dioca (nettle), Achillaea millefolium (yarrow); 1:1:1. In the study, in-vitro, the effect of pure herbal extract mixture on acid production of Streptococcus mutans was tested and the minimum inhibitory concentrations (MIC) of the following micro-organisms were tested: Streptococcus mutans, Streptococcus mitis, Actynomyces viscosus, Actynomyces naeslundii, Actinobacillus actinomycetemcomitans, Prevotella intermedia, Campylobacter rectus, Fusobacterium nucleatum, Veillonella parvula. The MIC-values for A. viscosus and P. gingivalis were 100 mg/ml. The MIC-values for A. naeslundii and A. actinomycetemcomitans were considerably lower (10 mg/ml). S. mitis was the most susceptible of the tested organisms to the extract with a MIC value of 1 mg/ml. S. mutans, C. rectus, V. parvula, and F. nucleatum were not influenced by the extracts. No inhibitory effect of the 6.3 mg/ml herbal extract mixture was observed on the acid production of S. mutans. For the study in-vivo, 45 volunteers were selected on the basis of having moderate gingival inflammation. As efficacy parameters the plaque index, modified gingival index and angulated bleeding index were assessed. The subjects were randomly divided among 3 experimental groups (2× test and 1‘minus active’control). The participants were requested to rinse with 10 ml of mouthwash twice a day for a period of three months. After 6 weeks and 3 months, the same clinical indices as at baseline were recorded. The results show no difference between the two test groups and the control group. In conclusion, the results of the present study have shown that the mixture of the 3 herbal extracts, Juniperus communis, Urtica dioca and Achillaea millefolium when used in a mouthrinse has no effect on plaque growth and gingival health.