The effect of zirconocenes, used as drugs for various therapeutic applications, on the crystal growth of hydroxyapatite was investigated at sustained supersaturation. AU zirconocenes were found to inhibit crystal growth of hydroxyapatite possibily through adsorption onto the active sites for crystal growth. The kinetics results favored a Langmuir-type isotherm, suggesting a high affinity of zirconocene dichloride for hydroxyapatite.