The objective of this research was to investigate the comparative in vitro enamel remineralisation potential of commercial toothpastes containing bioactive glass (BG) particles, hydroxyapatite (HAP) particles or casein phosphopeptide - amorphous calcium phosphate (CPP-ACP) nanocomplexes. Eighteen extracted permanent teeth were coated with varnish leaving a window on the buccal surface and placed in demineralising solution for 24 h to create artificial caries-like white spot lesions (WSLs). The teeth were randomly assigned to six groups and sectioned longitudinally through the WSLs. The roots were removed and the teeth were re-varnished, leaving the WSLs exposed. Groups A, B and C were subjected to an optimum remineralisation protocol in which the "control" half of each tooth was incubated in artificial saliva for 24 h at 37 degrees C and the "treatment" half of each corresponding tooth was cyclically exposed to artificial saliva and to 1: 2 toothpaste solution containing either BG, HAP or CPP-CAP, respectively. Groups D, E and F were subjected to an acid-challenge remineralisation protocol which was similar to that of Groups A, B and C but which also incorporated cyclic exposure to demineralising solution. Scanning electron microscopy and energy dispersive X-ray analysis were used to compare the remineralisation of the surface and depth of the control and treatment WSLs. Under optimum conditions BG and CPP-ACP provided sub-surface repair by diffusion of calcium and phosphate ions into the WSLs. HAP did not influence remineralisation under neutral pH conditions. Conversely, under acid-challenge conditions, HAP was able to dissolve to release calcium and phosphate ions which diffused in to the WSLs and also protected the enamel surface from further erosion. BG and CPP-ACP both coated the enamel surface under acidic conditions, although their ability to remineralise the body of the lesion was compromised at low pH.