The pathological process of Alzheimer's disease is closely related to amyloid fibril formation by the causative protein, amyloid beta (A beta). The growth behavior of A beta fibrils is predominated by the seeds-monomeric A beta interaction. In this study, the local hydrophobicity of seeds of A beta fibrils was investigated by the aqueous two-phase partitioning method to evaluate the hydrophobic interaction between the seed-monomeric A beta. The seeds showed a high local hydrophobicity relative to the monomer and fibrils. From the fibril growth experiment and the additive effect of Triton X-100, we could demonstrate the contribution of the hydrophobic seeds-monomer interaction to fibril formation.