To ensure that Aβ production was suppressed in concert with the dox-mediated inhibition of its precursor APPswe/ind, we measured Aβ40 and Aβ42 levels by ELISA in forebrain homogenates from young tet-off animals. At 1 mo of age, the mice lacked visible amyloid aggregates that might act as an intractable reservoir of peptide remaining in the brain after the transgene had been suppressed. To further ensure we could detect any such insoluble aggregates that might bias our measure of changes in peptide synthesis, we performed a sequential three-step extraction with PBS, 2% SDS, and 70% FA that would separate peptides by solubility. We compared the levels of human transgene-derived Aβ40 and Aβ42 in untreated mice at 4 and 6 wk of age to animals that had either been born and raised on dox or that had been left untreated for 4 wk and then placed on dox chow for 2 wk prior to harvest (the same groups described above for immunoblot analysis of APPswe/ind levels, line 107). Consistent with the reduction in full-length APPswe/ind synthesis shown by immunoblot (see Figure 1), we found that transgene-derived Aβ levels were completely suppressed in animals born and raised on dox, and were sharply reduced following acute (2 wk) antibiotic treatment. Compared to the levels in untreated 4-wk-old mice, PBS-soluble Aβ42 dropped by 95.2% following 2 wk of dox treatment and by 99.2% with chronic treatment (Figure 2A). Similarly, SDS-soluble Aβ42 decreased by 75.2% and 94.8% following 2-wk or lifelong dox treatment (Figure 2B). Only the FA fraction revealed a small dox-resistant pool of peptide in acutely treated animals that we believe represents stable predeposit aggregates that have already accumulated by 4 wk of age when treatment was begun (Figure 2C). Indeed, animals that were born and raised on dox did not harbor this reservoir of treatment-resistant peptide, with 96.3% less Aβ42 than untreated 4-wk-old mice. Measurement of total Aβ in chronically treated mice, including endogenous and transgene-derived peptide, demonstrated that Aβ levels in tet-off APP mice were reduced to the level of endogenous peptide found in nontransgenic animals (Figure 2D). Taken together with the immunoblotting data for full-length APPswe/ind, the ELISA measurements indicate that dox-mediated suppression of transgenic APPswe/ind synthesis leads to parallel reduction of Aβ levels.