Zinc salts have previously been shown to possess antiplaque activity in solution but this activity has as yet not been reported for dentifrices. The purpose of the present investigation was to establish the effect of dentifrices containing zinc citrate (ZCT) on plaque growth and on the concentration of zinc in saliva and plaque. Plaque growth inhibition was studied over periods of time approximating to intervals for normal oral hygiene procedures. Zinc in saliva and plaque was measured by atomic absorption spectroscopy. Elevated zinc levels in saliva were observed for 3-4 h after brushing with the zinc citrate dentifrice. Similarly, raised levels of zinc were encountered in plaque 1 h following contact with a slurry of a dentifrice containing ZCT. Increased concentrations of zinc were observed in plaque fluid and plaque residue. The plaque growth data from 88 participants in 5 studies revealed that plaque regrowth over 16 h or 22 h was reduced by dentifrices containing 0.5% ZCT, compared to placebo dentifrices. Dentifrices which contained 0.5% ZCT or 0.4% stannous fluoride, had similar inhibitory effects on plaque. Further analysis of the data showed that subjects with high rates of plaque growth benefited more from the zinc dentifrices than those with low rates of plaque growth. It is suggested that the effect on plaque was related to zinc concentration in the dentifrice rather than total dose applied, and that the main effect of zinc was to inhibit the extension of existing plaque.