The formation and growth of insoluble amyloid deposits composed primarily of the human beta-amyloid peptide (A beta) in brain is an essentially invariant feature of Alzheimer's disease (AD) and is widely believed to contribute to the progressive neurodegeneration of the disorder. To probe the specificity of amyloid formation and growth, we synthesized and examined the self-assembly of D- and L-stereoisomers of A beta in vitro. While both enantiomers formed insoluble aggregates at similar rates with amyloid-like fibrillar morphology, deposition of soluble A beta peptide onto preexisting A beta aggregates was stereospecific. Although the L-peptide deposited readily onto immobilized L-A beta aggregates with first-order kinetic dependence on soluble peptide concentration, essentially no association between the D-peptide and L-template was observed. Similarly, the D-peptide deposited with first-order kinetics onto a D-A beta aggregate template but did not deposit onto a similar template composed of aggregates of the L-enantiomer. Furthermore, although the L-A beta isomer deposited onto authentic AD amyloid in preparations of unfixed AD brain, no focal association between the D-peptide and brain amyloid was detected. These results establish that deposition of soluble A beta onto preexisting amyloid template is stereospecific, likely involving direct docking interactions between peptide backbone and/or side chains rather than simple hydrophobic association.