The invention involves identification of a mechanism of -amyloid peptide cytotoxicity, which enables treatment of conditions caused by -amyloid peptide aggregates by administration of compounds which antagonize the mechanism of cytotoxicity. The invention includes the identification and isolation of compounds which can antagonize the aggregation of -amyloid peptides and the neurotoxic effects of such aggregates. The compounds include isolated peptides which were selected for their ability to form a complex with a -amyloid peptide, or are derived from peptides so selected. Methods for treating conditions resulting from neurotoxic -amyloid peptide aggregates and pharmaceutical preparations are provided. Also provided are methods for selecting additional compounds which can antagonize the aggregation of -amyloid peptides and the neurotoxic effects of such aggregates.