Potential mechanisms for formation of highly organized biomineralized structures include oriented crystal growth on templates, the aggregation of nanocrystals by oriented attachment, and the assembly of inorganic nanoparticles mediated by organic molecules into aggregated structures. In the present study, the potential role of amorphous calcium phosphate (ACP) in facilitating the assembly of hydroxyapatite (HAP) nanoparticles into highly ordered structures was evaluated. The physical characteristics of HAP nanoparticles prepared by three different methods were analyzed after extended exposure to additives in solution. Higher order HAP architecture was detected only when the starting particles were aggregates of nanospheres with HAP cores and ACP shells. Enamel-like HAP architecture was produced when the biologic additive was 10 mM glycine or 1.25 microM amelogenin. Large platelike crystals of the type present in bone were induced when the additive was 10 mM glutamic acid. Surface ACP initially links the HAP nanoparticles in a way that allows parallel orientation of the HAP nanoparticles and then is incorporated into HAP by phase transformation to produce a more highly ordered architecture with features that are characteristic for HAP in biologic structures. These studies provide evidence for a new mechanism for assembly of biominerals in which ACP functions by linking HAP nanocrystals while they assume parallel orientations and then is incorporated by phase transformation into HAP molecules that rigidly link HAP nanocrystals in larger fused crystallites. Biologic molecules present during this process of biomineral assembly specifically regulate the assembly kinetics and determine the structural characteristics of the final HAP architecture.