Longo Vital^® herbal tablets have been shown to have a protective effect against periodontal bone loss in rats. This may be ascribed either to a previously demonstrated immuno-stimulatory effect of the tablets, to an antimicrobial effect of the herbs or to a combination of both. In the present study the in vitro susceptibility of 12 dental plaque bacteria to six individual herbs included in Longo Vital^® was determined by a broth dilution method. Paprika, rosemary leaves and peppermint inhibited two thirds of the tested bacteria at 2.8-45 mg/ml, 0.75-12 mg/ml and 3-24 mg/ml corresponding to 0.8-12.5 per cent, 1.6-25 per cent and 12.5-100 per cent of the recommended daily dose, respectively. A combination of paprika and rosemary leaves tested towards five susceptible bacteria revealed a decreased inhibitory effect on two of these bacteria, especially of Actinobacillus actinomycetemcomitans to paprika, but conversely a pronounced increase in susceptibility of Peptostreptococcus anaerobius, Porphyromonas gingivalis and Prevotella intermedia now susceptible to 0.01-0.70 mg/ml of each herb, corresponding to 0.02-0.2 per cent of the recommended daily dose. The active ingredients of the herbs, however, and the concentrations of these obtained in blood and periodontal tissues after ingestion of Longo Vital^® are not known.