Aggregation of beta-amyloid protein (Abeta) to form oligomers is considered to be a key step in generating neurotoxicity in the Alzheimer's disease brain. Agents that bind to Abeta and inhibit oligomerization have been proposed as Alzheimer's disease therapeutics. In this study, we investigated the binding of fluorescein-labeled Abeta(1-42) (FluoAbeta(1-42)) to SH-SY5Y neuroblastoma cells and examined the effect of the 39-kDa receptor-associated protein (RAP), on the Abeta cell interaction. FluoAbeta(1-42) bound to the cells in a punctate pattern. Surprisingly, when RAP was added to the incubations, FluoAbeta(1-42) and RAP were found to be co-localized on the cell surface, suggesting that RAP and Abeta may bind to each other. Experiments using the purified proteins confirmed that a RAP-Abeta complex was stable and resistant to sodium dodecyl sulfate. RAP also inhibited Abeta oligomerization. We next examined whether RAP could inhibit the neurotoxic effects of Abeta. Addition of Abeta(1-42) to SH-SY5Y cells caused an increase in intracellular Ca2+ that was inhibited by treatment of the Abeta peptide with RAP. RAP also blocked an Abeta-induced inhibition of long-term memory consolidation in 1-day-old chicks. This study demonstrates that RAP binds to Abeta and is an inhibitor of the neurotoxic effects of Abeta.