The physiological functions of the mouse telomeric major histocompatibility complex (MHC) class I molecules, including Hmt, are unknown. Hmt presents a polymorphic, N-formylated peptide encoded by the mitochondrial gene ND1 forming the cell surface maternally transmitted antigen (Mta). Because the N-formyl moiety is required for Hmt binding, we proposed that Hmt may function generally in presentation of N-formylated antigens. This hypothesis was validated by a competitive binding assay, demonstrating that synthetic N-formyl peptides from other mitochondrial genes also bound Hmt. Bacteria similarly initiate protein synthesis with N-formylmethionine; indeed, we established that Hmt can also present prokaryotic peptides in an N-formyl-dependent manner. These results indicate biochemical specialization of this MHCpeptide interaction and suggest a unique role for Hmt in prokaryotic host defenses. © 1991, Rockefeller University Press., All rights reserved.