Intact cells of Streptococcus mutans K-1 were incubated with enamel powder in buffer, pH 5.8, saturated with enamel salts, the cells or the enamel having been previously labelled with 32P. The cells were also incubated with p-nitrophenyl phosphate at varying pH or at pH 4.8 with varying concentrations of fluoride. Incubation of the K-1 cells with 32P-labelled enamel caused an equilibration of enamel 32P within the cells. During sucrose fermentation, the uptake of enamel 32P by the K-1 cells was rapidly increased several times higher than that of the controls. In the presence of fluoride, there was an increased liberation of bacterial 32P and the uptake of this by enamel was doubled. There were changes in the bacterial total phosphate corresponding to the fermentative accumulation of 32P of enamel origin within the cells or to the fluoride-induced release of 32P out of them. The acid phosphatase of intact K-1 cells had a maximum activity at pH 4.8 and a lower peak at pH 5.8. At pH 4.8, a fluoride concentration of 20 ppm inhibited this activity almost to zero. The results emphasize the phosphate translocation between enamel and bacteria as a common denominator in the enamel destruction and repair, In the former case this and the acid phosphatase may support the acid attack. The results with fluoride may partly explain its caries-inhibitory functions.