Aβ species including oAβ are present in both extracellular and intracellular fractions from young TgCRND8 mouse brains. Levels of human Aβ40 (A, B), human/rodent Aβ42 (C, D), and oAβ (E, F) were measured in the Tris-soluble, extracellular enriched fraction and the RIPA-extracted intracellular vesicle fraction from 2-mo-old (preamyloid deposition) TgCRND8 mouse brains (n = 10). Identically prepared extracts from wild-type (wt) littermates (n = 5) were included as controls to validate Aβ measurements in transgenic mice. For every assay, the signal in TgCRND8 transgenic mice was significantly higher than in wild-type mice. **P < 0.01, ***P < 0.001, ****P < 0.0001.