Interaction of bone and dentin proteins with minerals is an elementary step in the regulation of mineralization in these tissues. Adsorption of acidic non-collagenous proteins on hydroxyapatite was examined using fluorescence-labeled protein and synthetic hydroxyapatite. Phosphophoryn, bone Gla protein, osteonectin and bone small proteoglycan II were prepared and labeled with fluorescein. All of these proteins were adsorbed on hydroxyapatite with a dissociation constant on the order of 10(-7) M. The more acidic proteins had lesser binding capacities. Hydroxyapatite single crystals were incubated with labeled proteins and observed with a fluorescence microscope. Phosphophoryn and other acidic proteins were adsorbed preferentially on the (100) face of the crystal. This preferential adsorption of the acidic proteins may be responsible for the morphogenesis of biological hydroxyapatite.