We have studied the effects of dentin proteins, of Transforming Growth Factor beta 1 (TGF beta 1) and Bone Morphogenetic Protein (BMP2) on the differentiation of odontoblasts in vitro. The total EDTA-soluble fraction of dentin proteins, prepared from rabbit incisors was further separated by chromatography on DEAE-Cellulose and heparin-agarose columns. While the total EDTA-soluble fraction of dentin had no effect on cultured dental papillae, fractions retained on both columns were able to initiate functional differentiation of preodontoblasts of isolated day-17 first lower mouse molar dental papillae cultured in vitro. TGF beta 1 and BMP2, both stimulated the matrix secretion by dental papillae cells. TGF beta 1 and BMP2, combined with the inactive total EDTA-soluble fraction, stimulated odontoblast differentiation. An active fraction retained on DEAE-Cellulose completely lost the inductive activity after incubation with a neutralizing anti-TGF beta antibody. These results demonstrate that a TGF beta-like molecule present in dentin could interact with some component which acts as a modulator of its activity on the initiation of the cytological and functional differentiation of odontoblasts.