A double-blind cross-over study was performed to evaluate the inhibitory activity of silver zeolite (SZ) mouthrinse on plaque formation. Eleven dental students participated in this study. SZ mouthrinse was prepared by suspending SZ powder into phosphate-buffered saline (PBS) at a concentration of 3% (w/w). Type-A zeolite was used as a placebo. Before the experiment, the subjects were rendered plaque-free by professional prophylaxis. They then suspended any oral hygiene for five days, during which time they rinsed with either SZ or type-A zeolite mouthrinse twice a day. SZ significantly reduced plaque formation compared to the placebo (p < 0.05), suggesting that silver ions released from the SZ inhibited plaque formation.