We previously noted that some aged human cortical specimens containing very low or negligible levels of amyloid β-protein (Aβ) by enzyme immunoassay (EIA) provided prominent signals at 6∼8 kd on the Western blot, probably representing sodium dodecyl sulfate (SDS)-stable Aβ dimer. Re-examination of the specificity of the EIA revealed that BAN50- and BNT77-based EIA, most commonly used for the quantitation of Aβ, capture SDS-dissociable Aβ but not SDS-stable Aβ dimer. Thus, all cortical specimens in which the levels of Aβ were below the detection limits of EIA were subjected to Western blot analysis. A fraction of such specimens contained SDS-stable dimer at 6∼8 kd, but not SDS-dissociable Aβ monomer at ∼4 kd, as judged from the blot. This Aβ dimer is unlikely to be generated after death, because (i) specimens with very short postmortem delay contained the Aβ dimer, and (ii) until 12 hours postmortem, such SDS-stable Aβ dimer is detected only faintly in PDAPP transgenic mice. The presence of Aβ dimer in the cortex may characterize the accumulation of Aβ in the human brain, which takes much longer than that in PDAPP transgenic mice.