The conditions under which β-amyloid (Aβ) is toxic to primary rat hippocampal neurons were investigated. Synthetic Aβ(1–42) peptide was neurotoxic following “aging” for 7 to 14 days at 37°C in modified Eagle's media. Neurotoxicity included decreases in neurite length, cell number, and metabolic state. In contrast, aging Aβ(1–42) in the presence of the media supplement B27 inhibited Aβ(1–42)-induced neurotoxicity. Differences in the aggregation state of the two preparations did not account for differences in the biological activities elicited by each peptide. Since components of B27 include antioxidants as well as other agents that provide protection against oxidative damage, we suggest that free radicals may be responsible, in part, for the toxicity that occurs following the aging of the peptide.