Salivary proteins influence the biomineralization of hydroxyapatite (HAp) within enamel. Their effect on the crystal growth has been extensively studied, but, their effect on demineralization kinetics is less well investigated. In this study bovine serum albumin (BSA) was used as a model protein to measure its effect on demineralization kinetics of hydroxyapatite aggregates using scanning microradiography (SMR). HAp aggregates (8 and 20% porous) were cut into 5 x 5 x 2 mm blocks. SMR cells were prepared containing hydroxyapatite blocks. BSA was added to demineralising solutions (0.1 mol L(-1) acetic acid, buffered to pH 4.0; degree of saturation zero and 0.062 respectively) at a concentration range 0.76-75.8 micromol L(-1). Demineralising solution without added BSA was used as a control. The demineralising solutions were circulated past the samples at 0.4 mL min(-1). SMR was used to measure the rate of mineral loss (RML(HAp)) at 14 points in each sample repeatedly for 3 weeks. The results show that BSA increases or decreases the RML(HAp) depending on; BSA concentration, HAp porosity, and the degree of saturation of the demineralising solution. It is suggested that BSA influences demineralization kinetics of HAp either by modifying solution properties, or, by affecting the surface energy of hydroxyapatite.