The effect of acetaminophen; a widely used analgesic and fever reducing medicine; in supersaturated solutions of calcium phosphate was investigated under plethostatic conditions, at 37 degrees C, 0.15 M NaCl, pH 7.40. The rates of crystal growth measured in the presence of acetaminophen 1.654 x 10(-4) mol dm(-3) to 6.616 x 10(-4) mol dm(-3) were reduced by 43% to 79%, respectively. The inhibition effect on the crystal growth rate may be explained through adsorption onto the active growth sites. Kinetic analysis suggested Langmuir-type adsorption of acetaminophen on the HAP surface with a affinity value of 2.4 x 10-4 dm(3) mol(-1), for the substrate in the concentration range investigated. The electrophoretic mobility measurements showed that in the presence of acetaminophen the charge of the acetaminophen covered HAP particles was shifted to more negative values as compared to bare HAP. In the presence of acetaminophen no changes observed in the HAP overgrown morphology or in the apparent order of crystallization. (c) 2006 Elsevier B.V. All rights reserved.