Whole roots of molars that had never been exposed to the mouth were exposed to calcium and phosphate-containing buffer solutions with discrete values of pIOHA [i.e. -log(a10Ca2+ X a6PO4(3-) X a2OH-)] at various pH. From densitometric measurements on contact-microradiographs of transverse sections of the roots the rate of demineralization (Vdem) was calculated. Vdem changed non-linearly as a function of pIOHA and became zero at a pIOHA-value of 105.3 +/- 0.4; this is substantially lower than the corresponding value of 118 +/- 1 for enamel. Thus root mineral is more soluble than enamel mineral. The critical pH for root caries appears to be about 6.7, provided that the plaque fluid follows about the same pH-pIOHA- pathway as acidified saliva.