The aim of the present study was to assess the relative importance of the metal and fluoride components in the inhibiting effect on the acidogenicity of plaque exhibited by silver fluoride and copper fluoride. The effect of these salts was therefore compared with that of silver nitrate and copper sulfate. Four subjects with a 3-day-old sucrose induced plaque received topical application of the test agents. Plaque pH-measurements, in the presence of sucrose, were performed prior to and at given time intervals after this application. No significant differences were observed in the inhibitory effect of 5 mM solutions of the copper and silver salts. These salts were all more effective than stannous fluoride in the first 4 h following application. Sodium fluoride was the least effective. All metals tested were found to be retained in plaque. The inhibitory activity appears to be carried by the cations. Other properties than retention alone may explain the higher effectiveness observed for silver and copper.