Objective: To observe if there is a threshold value for mercury conversion by mutans streptococcus from inorganic to organic mercury. Methods: Eight isolated mutans streptococcus strains were used throughout the study. Serial concentrations of mercury in water were added to tubes containing a bacteria solution and incubated for 24 hours, then concentrations of organic and inorganic mercury were measured. A tube containing mutans streptococcus without mercury in water served as the positive control; a tube without mutans streptococcus but with mercury in water served as the negative control. Results: When the inorganic mercury concentration in the mutans streptococcus solution was lower that 0.0167 µg/ml, mutans streptococcus was barely capable of converting inorganic mercury to organic mercury; when the mercury concentration reached 0.033 µg/ml or more, mutans streptococcus was able to convert some inorganic mercury into organic mercury. Conclusion: It was demonstrated that there was a threshold value for mercury conversion by mutans streptococcus.