The present study showed that the plaque inhibiting effect of aqueous solutions of stannous fluoride can be maintained in toothpastes. A commercial toothpaste containing 0.14 per cent of water so luble tin was effective whereas one containing 0.03 per cent showed no effect. The plaque inhibiting effect was shown in one study where the toothpastes were applied in cap splints and the plaque was scored after four days by the Plaque Index of Löe, and in another where the effect of locally applied pastes was monitored 24 hours later by the Gingival margin Plaque Index of Harrap. The present investigation supports the view that the stannous ion is essential in the plaque inhibition caused by stannous fluoride preparation. The excact mechanism is unknown but it is speculated that a change in the surface potential of oral bacteria through interaction with the stannous ion may be important.