At the concentrations normally found in the brain extracellular space the glial-derived protein, S100B, protects neurons against neurotoxic agents by interacting with the receptor for advanced glycation end products (RAGE). It is known that at relatively high concentrations S100B is neurotoxic causing neuronal death via excessive stimulation of RAGE. S100B is detected within senile plaques in Alzheimer's disease, where its role is unknown. The present study was undertaken to evaluate a putative neuroprotective role of S100B against Abeta amyloid-induced neurotoxicity. We treated LAN-5 neuroblastoma cultures with toxic amounts of Abeta25-35 amyloid peptide. Our results show that at nanomolar concentrations S100B protects cells against Abeta-mediated cytotoxicity, as assessed by 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and terminal deoxynucleotidyl transferase-mediated dUTP-fluorescein isothiocyanate nick end-labeling (TUNEL) experiments, by countering the Abeta-mediated decrease in the expression of the anti-apoptotic factor Bcl-2. This effect depends on S100B binding to RAGE because S100B is unable to contrast Abeta-mediated neurotoxicity in neurons overexpressing a signaling-deficient RAGE mutant lacking the cytosolic and transducing domain. Our data suggest that at nanomolar doses S100B counteracts Abeta peptide neurotoxicity in a RAGE-mediated manner. However, at micromolar doses S100B is toxic to LAN-5 cells and its toxicity adds to that of the Abeta peptide, suggesting that additional molecular mechanisms may be involved in the neurotoxic process.