The invention relates to a method for the in vitro diagnosis of Alzheimer's disease using a monoclonal antibody. Said antibody can bind at least to amino acids 12-16 of the β-amyloid peptide, specifically detecting the neuritic plaques which are characteristic of Alzheimer's disease, without detecting diffuse plaques which are not defining characteristics of the disease. Within the neuritic plaques, the monoclonal antibody can detect a different sub-group in the composition of the different deposited isoforms of the β-amyloid peptide, which is associated with the disease progression stage. In addition, the antibody can bind to isoforms of the β-amyloid peptide in biological fluids such as urine. As a result, the inventive monoclonal antibody, the cell lines that produce said antibody and compositions containing same can be used in the in vitro diagnosis of Alzheimer's disease and in determining the disease progression stage.