The effect of dentin phosphoprotein (DPP) over a wide concentration range on precipitation and crystallization of calcium phosphate was studied in supersaturated solutions at pH 7.8 and 25 degrees C with a gel matrix model. The protein was isolated and purified from dentin of unerupted pig teeth. Depending on the incubation conditions the protein showed opposite effects on the life-time as well as on the relative particle size of amorphous calcium phosphate (ACP). This may be due to different conformation states of the protein. In the presence of Mg2+ the inhibitory effect of the protein on the lifetime is pronounced. An inhibition of the amount of ACP precipitate and the crystal growth rate by the phosphoprotein seems to be independent of its conformation state.