Degradation of sodium monofluorophosphate (MFP) by selected oral streptococci, in vitro streptococcal plaque, natural dental plaque, and saliva, was investigated over the pH range 4–9, using a fluoride-ion-selective electrode. At 37 °C, optimum MFP hydrolysis occurred at approximately pH 5, but the level of hydrolysis activity varied depending on the streptococcal strain, plaque type, and incubation period. At the end of 23 h of incubation, the hydrolysis activity ranged from 0.23 μmol MFP decomposed/mg protein for Streptococcus sanguis plaque to 1.92 μmol MFP decomposed/mg protein for the Streptococcus salivarius cell suspension. On this basis, the hydrolysis activity of cell suspensions of S. salivarius ranked relatively the highest, followed by those of Streptococcus mitis and Streptococcus mutans, BHT. No significant differences were observed with the activities for MFP hydrolysis at both the 2- and 23-hour incubation periods between in vitro plaque and cell suspensions for S. mutans, 10449. Likewise, no significant difference in hydrolysis activity was generally observed between saliva from the rinsed and unrinsed mouth, however, the presence of 0.05% of cetylpyridinium chloride in saliva produced a significant change, reducing it to negligible levels compared to cont