Purpose: The aim of the present investigation was to study the effect of daily use of fluoride dentifrices containing various antimicrobial agents on mutans streptococci (MS) in saliva and approximal dental plaque. Materials and Methods: Fifty-nine healthy adolescents, 12-14 years old, undergoing orthodontic treatment with fixed appliances and harbouring high levels of MS in saliva and preferably also in interdental plaque, were randomly distributed into four groups, using dentifrices with: 1) zinc lactate (n = 16), 2) amine fluoride-stannous fluoride (n = 13), 3) triclosan (n = 15), and 4) no antimicrobial agent (control; n = 15). Changes of MS scores versus baseline were determined after 1, 3 and 6 months, using the Dentocult® SM Strip mutans test. Results: At the 6-month sampling occasion, the subjects using dentifrice with either amine fluoride-stannous fluoride or triclosan showed a tendency to lower MS scores in interdental plaque (p 0.05). In saliva and in the 1- and 3-month plaque samples, no changes of MS were detected in any of the four groups. Conclusion: This 6-month clinical study showed that dentifrices with various antimicrobial agents only result in small or no changes of the MS scores in saliva and approximal dental plaque in orthodontic patients. Palabras clave: amine fluoride, caries prevention, dentifrice, mutans streptococci, stannous fluoride, triclosan