Alzheimer's disease (AD) is characterized by the aggregation of misfolded proteins. Previously we reported activation of the unfolded protein response (UPR) in AD neurons. A potential source for UPR activation in AD neurons may be the increased levels of β-amyloid (Aβ). In this study, we used preparations enriched in oligomeric or fibrillar Aβ1-42 to investigate the role of the conformational state of Aβ in UPR activation in differentiated neuroblastoma cells. Both oligomeric and fibrillar Aβ1-42 do not induce BiP expression to the extent that it can be detected in a pool of cells. However, using a fluorescent UPR reporter cell line that allows analysis of individual cells, we demonstrated mild activation of the UPR by oligomeric but not fibrillar Aβ1-42. We showed that oligomeric Aβ1-42 is significantly more toxic to cells primed for UPR than is fibrillar Aβ1-42, indicating that activation of the UPR contributes to oligomer-specific Aβ1-42 toxicity. Because UPR activation is observed in AD brain at a stage that precedes the massive fibrillar Aβ deposition and tangle formation, this may indicate a role for nonfibrillar Aβ in the induction of the UPR in AD neurons. © 2007 Mary Ann Liebert, Inc.