THE mutant human cell line T2 is defective in antigen presentation in the context of class I major histocompatibility complex (MHC) molecules^1,2 and also in that transfected T2 cells show poor surface expression of exogenous human class I (HLA) alleles³. Both defects are thought to lie in the transport of antigenic peptides derived from cytosolic proteins into the endoplasmic reticulum (ER)^1,2 as peptide-deficient class I molecules might be expected to be either unstable or retained in the ER⁴. The products of several mouse class I (H–2) genes, and the endogenous gene HLA-A2 do, however, reach the surface of T2 cells at reasonable levels although they are non-functional^3,5,6. We report here that, as expected, poorly surface-expressed HLA molecules do not significantly bind endogenous peptides. Surprisingly, H–2 omolecules expressed in T2 also lack associated peptides, arguing that 'empty' complexes of mouse class I glycoproteins with human β2-microglobulin are neither retained in the ER nor unstable. HLA-A2 molecules, however, do bind high levels of a limited set of endogenous peptides. We have sequenced three of these peptides and find that two, a 9-mer and an 11-mer, are derived from a putative signal sequence (of IP-30, an interferon-γ-inducible protein⁷), whereas a third, a 13-mer, is of unknown origin. The unusual length of two of the peptides argues that the 9-mers normally associated with HLA-A2 molecules may be generated before their transport from the cytosol rather than in a pre-Golgi compartment. To our knowledge, this is the first report of the isolation of a fragment of a eukaryotic signal peptide generated in vivo.