“Cross-priming” describes the activation of naïve CD8⁺ T cells by professional antigen-presenting cells that have acquired viral or tumor antigens from “donor” cells. Antigen transfer is believed to be mediated by donor cell–derived molecular chaperones bearing short peptide ligands generated by proteasome degradation of protein antigens. We show here that cross-priming is based on the transfer of proteasome substrates rather than peptides. These findings are potentially important for the rational design of vaccines that elicit CD8⁺ T cell responses.