Proteins associated with the mineral phase of dentin are considered to have the potential to alter cell function within the local environment, during development and regeneration of tooth/periodontal tissues. Cells that may be altered include osteoblasts, ameloblasts, periodontal ligament cells, odontoblasts, and cementoblasts. However, specific factors within dentin controlling cell activity have not been elucidated. To investigate further the role of dentin proteins in regulating cell behavior, MC3T3-E1 cells, a mouse osteoprogenitor cell line, were exposed to guanidine/EDTA extracts of dentin (G/E-D) prepared from bovine teeth. Cells, with or without G/E-D (2 to 50 μg/ ml), were evaluated for proliferative activity and for mRNA expression of bone-as-sociated genes. Results indicated that G/E-D suppressed cell proliferation and caused striking morphological changes, including the conversion of cuboidal cells into fibroblastic, spindle-shaped cells. Markers of osteoblast differentiation, osteocalcin and bone sialoprotein mRNA were decreased, while osteopontin mRNA was enhanced in cells exposed to G/E-D. Since transforming growth factor beta (TGFβ1) has been reported to influence cells in a similar fashion, G/E-D were examined for the presence of and concentration of TGFβ using slot blot analysis and enzyme immunoassay (ELI-SA), respectively. These analyses demonstrated that G/E-D contained 6.6 ng/mg of TGFβ1. Next, cells were exposed to G/E-D in conjunction with anti-TGFβ1,2,3 antibody. When cells were exposed to antibody, G/E-D-mediated changes in morphology and gene expression were blocked. These results suggest that TGFβ1 and perhaps other factors in dentin can regulate cell behavior and, therefore, can influence development, remodeling, and regeneration of mineralized tissues. J Periodontol 1998;69:1247–1255.