Inheritance of the epsilon 4 allele of apolipoprotein (apo) E is associated with increased risk of Alzheimer's disease (AD) and with increased beta-amyloid peptide (A beta) deposition in the cortex. Apo E is a member of a family of exchangeable apos, characterized by the presence of amphipathic alpha-helical segments that allow these molecules to act as surfactants on the surface of lipoprotein particles. Two members of this family, apo E and apo J, have been shown to bind soluble A beta, and both are associated with senile plaques in the AD cortex. We now have studied the pattern of brain apo expression and found that five members of this class are present: apo A-I, A-IV,D,E, and J. By contrast, apos A-II, B, and C-II were not detectable. Immunohistochemistry revealed that, in addition to apo E and apo J, apo A-I immunostained occasional senile plaques in AD cortex. Immunoblot analysis showed no difference in the relative amounts of any of these apos in tissue homogenates of frontal lobe from AD or control patients. Comparison by APO E genotype showed no differences in the amount of apo E in brain among APO E epsilon 3/3, epsilon 3/4, or epsilon 4/4 individuals; however, a significant decrease in the amount of apo J was associated with the APO E epsilon 4 allele. No differences in apo J levels were detected in CSF samples of AD subjects. We propose that several members of the exchangeable apo family may interact with A beta deposits in senile plaques through common amphipathic alpha-helical domains. Competition among these molecules for binding of A beta or A beta aggregates may influence the deposition of A beta in senile plaques.