Amyloid beta peptides are bound rapidly in the plasma complicating an accurate assessment of their in vivo abundance by immunoassay procedures. The extent of A beta immunoassay interference was used to estimate the A beta binding capacity of purified plasma proteins, erythrocytes and whole plasma. Human serum albumin bound A beta peptides rapidly with a 1:1 stoichiometry and at physiological concentrations was capable of binding over 95% of an input of 5 ng/ml A beta. Purified alpha 2-macroglobulin was able to bind A beta peptides and at physiological concentration bound 73% of 5 ng/ml of A beta. Erythrocytes also sequestered the A beta peptides, showing a preference for binding A beta 1-42. Incubation of 5 ng/ml of A beta in plasma revealed that about 30% of the peptides were still detectable by immunoassay, presumably reflecting the binding of A beta peptides with albumin and other plasma molecules. Thus, our studies reveal that both the soluble and formed elements of the blood are capable of sequestering A beta peptides. To avoid underestimating plasma A beta values, we employed an improved column chromatography method under denaturing conditions to liberate A beta from its associations with plasma proteins. Quantification of A beta 40 and 42 levels in plasma from both normal and AD individuals after chromatography showed a large overlap between AD and control groups, despite the very large pool of A beta present in the AD brains. The potential origins of the plasma A beta pool are discussed.