We have previously developed a solid-phase extraction (SPE) procedure to enable the detection of beta-amyloid (Abeta) peptides in brain tissue from non-transgenic animals. We have now adapted these methods to enrich the Abeta fraction in cerebrospinal fluid (CSF) and plasma. Human CSF and plasma and Tg2576 mouse plasma were subjected to guanidine denaturation followed by SPE in 96-well cassettes. The resulting eluates could be concentrated significantly to enhance detection of low-abundance Abeta peptides by immunoassay. The concentrated eluates diluted in a linear fashion with consistent recovery between SPE columns. This technique was therefore used to facilitate quantification of Abeta1-X, 1-40, 1-42, and 1-38 peptides in normal human CSF and plasma samples. SPE sample preparation was also applied to the plasma of mice dosed peripherally with a monoclonal antibody raised against Abeta. When such samples were assayed directly, the presence of the systemically administered antibody interfered with the subsequent immunoassay, by preventing detection of antibody-bound Abeta. After subjecting plasma from antibody-treated animals to denaturation and SPE, the antibody-antigen complex was disrupted, and the Abeta fraction could be isolated from the antibody-containing fraction. Application of this method allowed for detection of a 100-fold increase in plasma Abeta1-40 following treatment of Tg2576 mice or wild type littermate control mice with Abeta40-specific monoclonal antibody 9TL. Given the availability of a variety of SPE matrices, we hypothesize that these methods could facilitate plasma antigen retrieval using multiple therapeutic antibody approaches.