The aim of the present study was to compare the effect on plaque growth and gingival response of Meridol, an amine/stannous fluoride solution, during a three-month investigation, with those of a placebo preparation as a negative control and a chlorhexidine solution as a positive control, in a double-blind study. After having their teeth professionally cleaned, 36 volunteers continued their usual oral hygiene for a period of two weeks. Their teeth were then polished again (month 0) after which they rinsed twice daily (morning and evening) with one of the three mouthrinses in addition to their habitual toothcleaning. After three months of rinsing, plaque indices remained lowest in the chlorhexidine group, although the subjects using Meridol had indices significantly lower than those of individuals rinsing with the placebo solution. The gingival index scores in the Meridol group were higher than in the chlorhexidine group and lower than in the placebo group. The plaque vitality scores showed a bactericidal effect in vivo with chlorhexidine and Meridol during the entire experiment. The use of Meridol resulted in more toothstaining than with the placebo, but significantly less than with chlorhexidine. This study demonstrated that Meridol reduced plaque accumulation, retarded gingivitis development, possessed a definite bactericidal action, and produced only slight toothstaining.