When diluted from dimethyl sulfoxide or 1,1,1,3,3,3-hexafluoro-2-propanol, synthetic human Aβ(1-42) readily forms oligomeric structures at near physiologic concentrations (1–20nM). Oligomers ⩾40kDa are detected in a sandwich enzyme-linked immunosorbant assay where the capture and detection antibodies recognize the same primary sequence epitope. Monomeric peptide with a single epitope does not react in this format. Aβ(1-40) peptide does not oligomerize readily under these conditions. The rate of oligomer formation has a steep linear temperature dependence but is weakly affected by ionic strength up to 0.5M NaCl or KCl. Oligomer formation is inhibited by concentrations of Tween 20 and several other detergents well below their critical micelle concentrations. Once formed, high-molecular-weight oligomers are stabilized by Tween 20. Gel permeation chromatography of an oligomer preparation formed at nanomolar concentrations indicates that the majority of the Aβ(1-42) peptide chromatographs as monomers/dimers of apparent mw ∼10kDa. The most abundant oligomers have apparent mobilities corresponding to 220kDa (48-mer) and higher multiples of this without detectable concentrations of intermediate low-molecular-weight species. Very little immunoreactive peptide appears in the void volume (>1.5MDa) of a Superose 12 column. The oligomers are stable, rechromatographing at their original position. Aβ(1-42) oligomer formation at physiologic concentrations is a reproducible process that is amenable to kinetic analysis and inhibition.