a new metalloendopeptidase was purified to apparent homogeneity from a homogenate of normal human brain using successive steps of chromatography on deae-trisacryl, hydroxylapatite and sephacryl s-200. the purified enzyme cleaved the gly33-leu34 bond of the 25-35 neurotoxic sequence of the alzheimer ？-amyloid 1-40 peptide producing soluble fragments without neurotoxic effects. this enzyme activity was only inhibited by divalent cation chelators such as edta, egta and o-phenanthroline (1 mm) and was insensitive to phosphoramidon and captopril (1 μm concentration), specific inhibitors of neutral endopeptidase (ec 22.214.171.124) and angiotensin-converting enzyme (ec 126.96.36.199), respectively. the high affinity of this human brain endopeptidase for ？-amyloid 1-40 peptide (km = 5 μm) suggests that it may play a physiological role in the degradation of this substance produced by normal cellular metabolism. it may also be hypothesized that the abnormal accumulation of the amyloid ？-protein in alzheimer's disease may be initiated by a defect or an inactivation of this enzyme.