The polymorphic protein, serum amyloid A (SAA), consists of acute phase isotypes and a constitutive isotype. Both are associated mostly with high density lipoproteins (HDL) in the circulation. In the present study, both SAA isotypes were detected by immunohistochemistry and immunoblotting using monoclonal antibodies in atherosclerotic lesions. As the distribution of SAA was identical with that of apolipoprotein B and SAA is known to be associated also with low density lipoproteins (LDL), SAA may also be delivered to the artery wall by LDL.