Neurons in the central nervous system (CNS) are vulnerable to radical stress caused by reactive oxygen species, including nitric oxide (NO). Those radicals play crucial roles in glutamate neurotoxicity associated with ischemic brain injury and a wide range of neurodegenerative disorders. In our previous studies, we have shown evidence suggesting that glutamate neurotoxicity is regulated by certain endogenous substances such as neurotrophins, nicotinic acetylcholine, prostanoids and vitamins. Based on those findings, we have used the term 'neuroprotective factor' for endogenous substances possessing protective activity against glutamate neurotoxicity, and have further searched for a candidate with unique structure. We isolated a novel neuroprotective substance named 'serofendic acid' derived from fetal calf serum. The compound exhibited potent protective action against neurotoxicity induced by glutamate and by an NO donor without inhibiting glutamate receptors. Electron spin resonance analysis demonstrated that serofendic acid had no direct scavenging activity on NO, but was capable of inhibiting the generation of a hydroxyl radical, a presumed 'executor' radical in the nitric oxide-mediated neurotoxic cascade. The chemical structure was determined by mass spectrometry and nuclear magnetic resonance spectroscopy, and was confirmed by synthesis. The structure was unique among known endogenous substances because the compound was a sulfur-containing atisane type diterpenoid. The discovery of serofendic acid may provide a new scope for the investigation of low-molecular weight bioactive factors promoting the survival of CNS neurons.