Antibodies that specifically bind to either amyloid-β peptide (Aβ) isoform Aβ40 or Aβ42 contribute to the study of Alzheimer's disease (AD) pathology and to the development of cerebrospinal fluid-based tests for the probable diagnosis of AD. Polyclonal rabbit anti-Aβ antibodies possess high affinity and specificity, but their generation requires a long immunization period, and the resulting antibodies exhibit variable specificities and affinities. To secure a continuing supply of antibodies with uniform properties, we generated and partially characterized rabbit monoclonal antibodies specific for either Aβ40 or Aβ42. These antibodies possess nanomolar or sub-nanomolar dissociation constants and are at least 3,000-fold more selective for one isoform over the other. These antibodies are suitable for immunoblotting and, in a sandwich ELISA, RabmAb42 (anti-Aβ42) is sensitive enough to measure plasma levels of Aβ42. In addition, these antibodies have been applied to the immunohistology of Down syndrome and AD brain tissues, where they reveal fibrillar and diffuse amyloid deposits and are almost free of non-specific staining. The data indicate that diffuse amyloid deposits contain only minute amounts of Aβ40. Thus these rabbit monoclonal anti-Aβ antibodies can be widely applied in AD and Down syndrome research and diagnosis.