The protective effects of non-steroidal anti-inflammatory drugs (NSAIDs) in Alzheimer's disease have been demonstrated in multiple epidemiological studies. It has been hypothesized that this is due to their effects on amyloid β-peptide (Aβ) metabolism, which is independent of the NSAID stereoisoform, rather than inhibition of cyclooxygenase (COX), which is a property of S-enantiomers. We compared the neuroprotective activity of S- and R-enantiomers of flurbiprofen and ibuprofen in a standard assay where secretions from activated human THP-1 or microglial cells are toxic to neuroblastoma SH-SY5Y cells. We found S- but not R-enantiomers to be protective at low concentrations, which is consistent with a COX-dependent mechanism.