The effect of Prothymosin alpha (Protalpha) fragments (used as drugs for various applications) in supersaturated solutions of calcium phosphate was investigated under plethostatic conditions. The rates of crystal growth measured in the presence of Protalpha were drastically reduced on the size of the fragment. This inhibitory effect may be explained by adsorption and subsequent blocking of the active growth sites. Kinetic analysis suggested Langmuir-type adsorption of Protalpha on the hydroxyapatite (HAP) surface with a relatively high affinity for the substrate in the concentration range investigated. In all cases, the measured crystal growth rates, R, were proportional to the relative supersaturation, S, with respect to HAP. Also, the rate of crystal growth was reduced upon increasing the size of the Protalpha fragment reaching a plateau, a fact that may be ascribed to further blocking of the active growth sites. (C) 2004 Elsevier B.V. All rights reserved.