Influence of Sr 2+ on the formation and structure of calcium hydroxyapatite (Ca 10 (PO 4) 6 (OH) 2 : CaHap) particles prepared by hydrolyzing the phenyl phosphate (C 6 H 5 PO 4 H 2) using alkaline phosphatase (ALP) at 37°C was examined. Increasing the additional amount of Sr 2+ increased the Sr content in the products and turned the crystal phase of the products as follow: CaHap  solid solution SrCaHap  Sr 1.7 Ca 1.3 (PO 4) 2  (3Sr(HPO 4) 2 •Sr 3 (PO 4) 2)•2H 2 O. All the phenyl phosphates were hydrolyzed by ALP at 37°C. The crystallinity and particle size of CaHap decreased with an increase of added Sr 2+ , meaning the inhibition of crystallization and particle growth. These facts suppose that incorporation of Sr 2+ in animal organism suppresses the formation of hard tissues.