The incidence of dental caries, and chemical, microbial and electron histochemical analyses of plaque and its acidogenic potential were determined in groups of monkeys fed a cariogenic diet for 3 years in which sucrose was supplemented with phytate or pyridoxine or replaced by invert sugar while the controls received sucrose alone. Only phytate affected a marked reduction in the incidence of caries which did not correlate with changes in plaque composition which suggests that phytate acts by reducing enamel solubility. Pyridoxine had a small but insignificant cariostatic effect; invert sugar was as cariogenic as sucrose.