The invention concerns a novel .beta.-secretase, a method of partially purifying this novel .beta.-secretase, and its use in assays to screen for potential drug candidates against Alzheimer's disease and other neurological diseases. The novel .beta.-secretase has an estimated molecular weight of about 32 39 kDa or 22 26 kDa in HEK293 cell membrane extracts and human brain samples, respectively, as calculated from radiation inactivation analysis, and has a pH optimum at about pH 6.5 7.0.