Transformation of amorphous calcium phosphate (ACP) to hydroxyapatite (HAP) and subsequent crystal growth of HAP were retarded in the presence of phosphorylated polyvinylalcohol (Phos.PVA) and phosphoserine (PSer), while the mother compounds, polyvinylalcohol (PVA) and serine (Ser), showed no specific effect on these factors over the concentration range investigated. The retardation was caused through the competitive adsorption between inorganic phosphate ion (Pi, one of the lattice ions) and the phosphate group of the organic compounds (i.e., Phos. PVA and PSer) for the active growth sites on the HAP crystal or nucleus. Phos. PVA was about 20 times stronger than PSer in its effect owing to the fact than the thick adsorption layer of the former repels Pi more efficiently than the thin adsorption layer of the latter. These results suggest that high molecular phosphorylated compounds such as phosphoproteins are more significant in regulation/retardation of biological mineralization/crystallization in mammalian body than low molecular phosphorylated compounds such as PSer.