Cross-priming is the process in which Ag-presenting dendritic cells (DCs) acquire, process, and present Ags scavenged from other cells, and use these cells to activate naive CD8 T cells. Cross-priming of cognate CD8 cells can result in either tolerance or immunity, depending upon the activation status of the Ag-presenting DC. Previous studies have shown that nominal peptide is inefficiently cross-presented and that proteins and large polypeptides that require proteasomal processing are the main source of naturally cross-presented Ags. In this study we show that N-terminal extension of nominal peptide by as few as three residues is sufficient to produce a substrate for TAP-dependent cross-presentation that is highly efficient in cross-priming murine CD8 T cells in vivo. On a molar basis, cross-priming with 3-mer-extended peptide is 20-fold more efficient than priming with intact protein. This method of peptide extension should prove of great value in facilitating in vivo studies of CD8 immunity and tolerance that rely on cross-presentation.