A key feature of Alzheimer's disease (AD) is the pathogenic self-association of the amyloid-β (Aβ) peptide, leading to the formation of diffusible Aβ oligomers and extracellular amyloid plaques. Today, more than Aβ fibrils, small soluble Aβ oligomers are suspected to be the major toxic species responsible for AD development and progression. Next to extracellular Aβ, intracellular Aβ might have important pathological functions in AD. Therefore, agents that specifically interfere with early oligomerization processes either outside or inside of neurons are highly desired for the elucidation of the pathologic mechanisms of AD. Such substances maybe even pave the way for new AD gene therapeutic approaches. In a phage display selection, we identified the Aβ oligomer binding and modulating peptide “L3”. Here, we characterize L3-Aβ binding and the influence of L3 on Aβ oligomerization in vitro using various biophysical methods. Preliminary studies in cell culture demonstrate that stably expressed L3 reduces cell toxicity of externally added and co-expressed Aβ in neuroblastoma (N2a) cells.