The dementia of Alzheimer disease (AD) correlates with the deposition of extracellular amyloid, and this amyloid arises from the abnormal processing of a high molecular weight amyloid peptide precursor (APP), which is a normal cellular protein that is found in both brain and in peripheral tissues in humans. Overproduction of the APP in AD could cause increased concentrations of this protein in either human blood or cerebrospinal fluid (CSF). However, thus far no direct demonstration of soluble APP in human blood has been possible, owing to poor assay sensitivity and interfering plasma proteins. These two problems were eliminated with the present development of an extracting two-site immunoradiometric assay (IRMA). Two rabbit polyclonal antisera were prepared reacting to two different sites (amino acids 161–180 and 597–624) of the APP molecule. The near N-terminal antiserum (anti-APP 161–180) was covalently coupled to a solid phase support and the near C-terminal directed antiserum (anti-APP 597–624) was indirectly labeled using 125labeled near C-terminal synthetic peptide corresponding to amino acids 597–624. The IRMA was validated by partial purification of the APP from human serum and demonstration of the protein's molecular weight (112 kDa) by Western immunoblot procedures. Results of the IRMA showed that the APP is present in human plasma (mean ± SE concentration=32 ± 6 pM, n=25), and there was no significant difference in the APP concentration in 25 controls, 19 patients with AD, and 10 individuals with Down syndrome (DS). Immunoreactive APP was generally not detectable in control or AD CSF volumes as large as 1 ml. In conclusion, these studies provide the first demonstration of the presence of APP in human blood that is immunoreactive with an antiserum directed against the amyloidotic portion (i.e., amino acids 597–624) of the APP molecule. In addition, these studies show that soluble APP is present in human blood, but that its concentration is not increased in AD.