Bone sialoprotein (BSP) is a bone-specific glycoprotein containing phosphoserine and sulphotyrosine residues and regions of contiguous glutamic acid residues. Recent studies in this laboratory have shown that BSP is capable of nucleating the bone mineral hydroxyapatite in a steady-state agarose gel system. We show here that chemical modification of carboxylate groups abolishes the nucleation activity of BSP, but enzymic dephosphorylation has no effect. Formation of hydroxyapatite is also induced by poly(L-glutamic acid) and poly(D-glutamic acid), but not by poly(L-aspartic acid) or poly(L-lysine). Calreticulin, a muscle protein with short sequences of contiguous glutamic acid residues, also lacks nucleation activity. These findings suggest that the nucleation of hydroxyapatite by BSP involves one or both of the glutamic acid-rich sequences. Based on these findings and others, we propose that polycarboxylate sequences represent a general site for growth-modulating interactions between proteins and biological crystals.