Peptide aldehyde inhibitors of the chymotrypsin-like activity of the proteasome (CLIP) such as N-acetyl-Leu-Leu-Nle-H (or ALLN) have been shown previously to inhibit the secretion of β-amyloid peptide (Aβ) from cells. To evaluate more fully the role of the proteasome in this process, we have tested the effects on Aβ formation of a much wider range of peptide-based inhibitors of CLIP than published previously. The inhibitors tested included several peptide boronates, some of which proved to be the most potent peptide-based inhibitors of β-amyloid production reported so far. We found that the ability of the peptide aldehyde and boronate inhibitors to suppress Aβ formation from cells correlated extremely well with their potency as CLIP inhibitors. Thus, we conclude that the proteasome may be involved either directly or indirectly in Aβ formation.