Endoplasmic reticulum (ER) aminopeptidase 1 (ERAP1) appears to be specialized to produce peptides presented on class I major histocompatibility complex molecules. We found that purified ERAP1 trimmed peptides that were ten residues or longer, but spared eight-residue peptides. In vivo, ERAP1 enhanced production of an eight-residue ovalbumin epitope from precursors extended on the NH2 terminus that were generated either in the ER or cytosol. Purified ERAP1 also trimmed nearly half the nine-residue peptides tested. By destroying such nine-residue peptides in normal human cells, ERAP1 reduced the overall supply of antigenic peptides. However, after interferon-gamma treatment, which causes proteasomes to produce more NH2-extended antigenic precursors, ERAP1 increased the supply of peptides for MHC class I antigen presentation.