The inheritance of the apolipoprotein E (apoE) ϵ4 allele is a prevailing risk factor for sporadic and familial Alzheimer's disease (AD). ApoE isoforms bind directly to Alzheimer's amyloid β (Aβ) peptides both in vitro and in vivo. Recent studies suggest that association of apoE with lipids may modulate its interaction with Aβ. We examined the binding of lipid-associated and delipidated apoE3 and apoE4 isoforms to Aβ utilizing a solid-phase binding assay and estimated the dissociation constants for the interaction of various apoE and Aβ species. Using native apoE isoforms from stably transfected RAW 264 and human embryonic kidney 293 cells, apoE3 had greater affinity than apoE4 for both Aβ1-40 and Aβ1-42. Delipidation of apoE decreased its affinity for Aβ peptides by 5-10-fold and abolished the isoform-specificity. Conversely, incorporation of apoE isoforms produced by baculovirus-infected Sf9 cells into reconstituted human high-density-lipoprotein lipoparticles restored the affinity values for Aβ peptides and resulted in preferential binding of apoE3. The data demonstrate that native lipid-associated apoE3 binds to Aβ peptides with 2-3-fold higher affinity than lipid-associated apoE4. Since the isoforms' binding efficiency correlate inversely with the risk of developing late-onset AD, the results suggest a possible involvement of apoE3 in the clearance or routing out of Aβ from the central nervous system as one of the mechanisms underlying the pathology of the disease.