The majority of cases of early-onset familial Alzheimer disease are caused by mutations in the recently identified presenilin 1 (PS1) gene, located on chromosome 14. Presenilin 1, a 467 amino acid protein, is predicted to be an integral membrane protein containing seven putative transmembrane domains and a large hydrophilic loop between the sixth and seventh membrane-spanning domain.Seven IgG monoclonal antibodies, termed MKAD3.1 to MKAD3.7, were isolated after immunization with a synthetic peptide, containing the residues 21 to 59 of the hydrophilic N-terminus of PS1. Epitope mapping, using the multipin method, showed that the antibodies can be divided in three groups according to their reactivity with 3 different non-overlapping epitopes. The specificity of the antibodies was confirmed by reactivity with bacterially expressed glutathione-S-transferase fusion proteins of full size presenilin 1.The antibodies were subsequently used to characterize PS1 protein in membrane extracts from human SH-SY neuroblastoma cells, human 293 kidney cells and human brain. In all three extracts the antibodies MKAD3.3 and MKAD3.7 were able to detect a weak band at M r 47,000 (47K), corresponding to full-size PS1. We prepared an affinity column using the monoclonal antibody MKAD3.3 and purified PS1 from SH-SY cells and from human brain. The purified proteins were evaluated by immunoblot with all seven monoclonal antibodies and with two different polyclonal rabbit antisera raised against sequences in the large hydrophilic loop and the C terminus of PS1. The 47K protein is recognized not only by all seven monoclonal antibodies but also by both polyclonal antisera confirming its identity as full size PS1. Staining by monoclonal antibody MKAD3.3 of cell-lysates from PC12 cells transfected with human PS1 also enabled us to confirm that PS1 is expressed as a 47K protein.