Amyloid beta 1-40 peptide was specifically isolated and analyzed from human plasma spiked with amyloid beta using a combined method of biotinylated anti-amyloid beta antibody binding to membrane-immobilized avidin (immunoaffinity membrane) and matrix-assisted laser desorption /ionization time-of-flight mass spectrometry (MALDI-TOF MS). A solution of 10 mu L containing 13.6 ng to 2.9 mu g of amyloid beta peptide was examined in this method. After the isolated amyloid beta peptide from the spiked human plasma containing 2.9 mu g of amyloid beta peptide was incubated in the presence of trifluoroacetic acid, fibrillization of the peptides was observed using a thioflavin T assay. Furthermore, an immunoaffinity membrane present on the inner wall of a tube (diameter 2 mm) captured the amyloid beta peptide from the spiked human plasma. Our results indicate that the combination of the immunoaffinity membrane procedure and MALDI-TOF MS can be used to capture and analyze the target antigens such as amyloid beta in micro-spaces.