Oligomeric species formed by the amyloid β-peptide (Aβ) are involved in the pathogenesis of Alzheimer’s disease. Eventually, Aβ forms protease resistant fibrils that are deposited in amyloid plaques, which together with intracellular deposits formed by the tau protein, are the hallmarks of AD. We have used laser pressure catapulting and liquid chromatography-mass spectrometry for purification and analysis of plaque cores. Only Aβ was found to be present in the plaque cores, indicating that no additional proteins are necessary for plaque formation. The C-terminus of Aβ is of importance for the polymerization process, and the 42 residue Aβ variant (Aβ42) has a strong tendency to form oligomers and fibrils. To clarify what Aβ variants that are deposited in the human brain, we purified plaque cores from sporadic- as well as familial AD-cases, and identified and quantified the Aβ variants present. Interestingly, a longer variant, Aβ43, was found to be present in most cases, and we suggest that this variant could be of importance for AD. Finally, by using laser capture microdissection to isolate pyramidal cells or purkinje neurons, and ELISA for quantification, we show that the intraneuronal levels of Aβ42 correlate with AD.