We analyzed the composition of amyloid beta protein (A beta) species in cerebral amyloid angiopathy (CAA) of an aged squirrel monkey. Immunocytochemistry demonstrated that the cerebral cortex contained no lesions other than widespread CAA with A beta40 as its apparent major component. However, enzyme-linked immunosorbent assay revealed that A beta42(43) predominated over A beta40 in a formic acid-extracted cortical fraction. These findings suggest possible underestimation of A beta42(43) levels in some previous immunocytochemical investigations.