The plaque-inhibiting capacity of 5-, 10-, 20-, and 100-mM ZnCl2 solutions was examined in 10 dental hygienist students. Twice daily mouthrinses with a 10-ml solution of 5, 10, and 20 mM ZnCl2 significantly inhibited plaque formation (p less than or equal to 0.05), whereas 100 mM ZnCl2 had only a negligible effect. Mouthrinses with 5 and 100 mM ZnCl2 were also tested in 6 dental students characterized as being heavy plaque formers. In this group, the antiplaque effect of 5 mM ZnCl2 was more pronounced, while 100 m ZnCl2 also significantly decreased plaque accumulation. The effect of a single mouthrinse with a 10-ml solution of 5 and 100 mM ZnCl2 on plaque acidogenicity was also assessed in 5 dental students. 5mM ZnCl2 significantly depressed acid production by plaque 30 min after a rinse, while 100 mM ZnCl2 inhibited acid production significantly for up to 4 h. 100 mM ZnCl2 was significantly more effective at depressing acid production than 5 mM ZnCl2 30 min after a rinse.