There is now extensive evidence that amyloid-β peptide is toxic to neurons and that its cytotoxic effects can be attributed to a domain corresponding to amyloid-β 25-35, GSNKGAIIGLM. We have shown recently that the serine proteinase inhibitor (serpin)-enzyme complex receptor (SEC-R), a receptor initially identified for binding of α1-antitrypsin (α1-AT) and other serine protease inhibitors, also recognizes the amyloid-β 25-35 domain. In fact, by recognizing the amyloid-β 25-35 domain, SEC-R mediates cell surface binding, internalization, and degradation of soluble amyloid-β peptide. In this study, we examined the possibility that SEC-R mediates the neurotoxic effect of amyloid-β peptide. A series of peptides based on the sequences of amyloid-β peptide and α1-AT was prepared soluble in dimethyl sulfoxide or insoluble in water and examined in assays for SEC-R binding, for cytotoxicity in neuronal PC12 cells and murine cortical neurons in primary culture, and for aggregation in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. The results show that amyloid-β peptide 25-35 and amyloid-β peptide 1-40 prepared soluble in dimethyl sulfoxide compete for binding to SEC-R, are nontoxic, and migrate as monomers in SDS- PAGE analysis. In contrast, the same peptides aged in water did not compete for binding to SEC-R but were toxic and migrated as aggregates in SDS-PAGE. An all-D-amyloid-β 25-35 peptide was not recognized at all by SEC-R but retained full toxic/aggregating properties. Using a series of deleted, substituted, and chimeric amβ/α1-AT peptides, toxicity correlated well with aggregation but poorly with SEC-R recognition. In a subclone of PC12 cells which developed resistance to the toxic effect of aggregated amyloid-β 25- 35 there was a 2.5-3-fold increase in the number of SEC-R molecules/cell compared with the parent PC12 cell line. These data show that SEC-R does not mediate the cytotoxic effect of aggregated amyloid-β peptide. Rather, SEC-R could play a protective role by mediating clearance and catabolism of soluble, monomeric amyloid-β peptide, if soluble amyloid-β peptide proves to be an in vivo precursor of the insoluble, toxic peptide.