The effects of two diphosphonates, disodium ethane-1-hydroxy-1,1 diphosphonate (EHDP), disodium methane diphosphonate (MDP) and NaF on the development of artificial caries were investigated quantitatively. An inhibition by both diphosphonates and NaF was quantitatively demonstrated. The effect of diphosphonates on the caries process differed from that of NaF. Whereas NaF inhibited the Ca and P release probably by the formation of fluorapatite, diphosphonates seemed to reduce caries by a reaction with the enamel, concomitant with a liberation of P to the cariogenic medium.