The amyloid β-protein (Aβ) is believed to play a causal role in Alzheimer's disease, however, the mechanism by which Aβ mediates its effect and the assembly form(s) of Aβ responsible remain unclear. Several APP transgenic mice have been shown to accumulate Aβ and to develop cognitive deficits. We have studied one such model, the J20 mouse. Using an immunoprecipitation/Western blotting technique we find an age-dependent increase in Aβ monomer and SDS-stable dimer. But prior to the earliest detection of Aβ dimers, immunohistochemical analysis revealed an increase in oligomer immunoreactivity that was coincident with reduced hippocampal MAP2 and synaptophysin staining. Moreover, biochemical fractionation and ELISA analysis revealed evidence of TBS and triton-insoluble sedimentable Aβ aggregates at the earliest ages studied. These data demonstrate the presence of multiple assembly forms of Aβ throughout the life of J20 mice and highlight the difficulty in attributing synaptotoxicity to a single Aβ species.