Aβ 1-42, 42-residue fragment of amyloid precursor protein, has been found to be a major constituent of the senile plaques formed in the brains of patients with Alzheimer’s disease and late Down’s syndrome. Aβ 1-42 readily forms neurotoxic oligomers at physiological pH. On the other hand, the peptide shows antimicrobial activity. The sequence of this peptide corresponds to the sequence of human, bovine, canine, feline, ovine, guinea pig, and rabbit Aβ42. The peptide has been used to detect amyloid β-protein multimers in the cerebrospinal fluid of Alzheimer’s disease patients through fluorescence correlation spectroscopy. For detailed descriptions of the preparation of Aβ 1-42 monomers and protofibrils please see the papers of Jan, Hartley, and Lashuel, Stine et al. (2011), and of Broersen and colleagues. The findings of Ryan et al. indicate that 10% ammonia disaggregates Aβ42 more efficiently than HFIP.