Three potent rabbit antisera to human serum amyloid A protein (SAA) appear to be directed exclusibely to the carboxy terminal region not shared with the tissue amyloid A protein. Since binding to albumin completely blocks the reaction of these antisera with the antigen, and since SAA exists in serum complexed to albumin, the anti-SAA cannot be used to detect or quantitate this serum component. The possibility that similar problems will be encountered with immunoassays for molecules that exist complexed to other proteins is discussed.

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