Abstract: Monoclonal antibody and the hybridoma producing the antibody are disclosed. The antibody is produced against an epitope on native secreted forms of .beta.APP. A characteristic of the antibody is that the antibody specifically binds to at least one protein in culture medium in which neuroblastoma cells have been grown. This protein co-migrates in non-reducing polyacrylamide gel electrophoresis with PN-2 isolated from human fibroblast cultures. Other characteristics of the antibody are that the antibody specifically binds to PN-2 and to the secreted form of .beta.APP lacking the Kunitz-type inhibitor domain, the antibody selectively binds to neuritic plaques, and the antibody is sufficiently sensitive to detect secreted forms of .beta.APP in CSF, down to a total concentration of the secreted forms of .beta.APP of 3.75 .mu.g/ml or lower.