Taking advantage of our understanding of the peptide specifidty of the major histocompatibility complex class I-b molecule M3a, we sought to determine why these molecules are poorly represented on the cell surface. To this end we constructed a chimeric molecule with the α1 and α2 domains of M3a and α3 of Ld thereby allowing use of available monoclonal antibodies to quantify surface expression. Transfected, but not control, B10.CAS2 (H-2M3b) cells were lysed readily by M3a-restricted monoclonal cytotoxic T lymphocytes. Thus, the chimera bound, trafficked, and presented endogenous mitochondrial peptides. However, despite high levels of M3a-Ld mRNA, transfectants were negative by surface staining. This finding was consistent with ineffident trafficking to the cell surface. Incubation at 26 °C, thought to permit trafficking of unoccupied heavy (H) chains, resulted in detectable cell surface expression of chimeric molecules. Incubation with exogenous peptide at 26 °C (but not at 37°C) greatly enhanced expression of M3a-Ld molecules in a dose-dependent manner, suggesting stabilization of unoccupied molecules. Stable association of β2-microglobulin with the chimeric H chain was observed in labeled cell lysates only in the presence of exogenous specific peptide, indicating that peptide is required for the formation of a ternary complex. These results indicate that surface expression of M3a-Ld is limited largely by the steady-state availability of endogenous peptides. Since most known M3a-binding peptides are N-formylated, native M3a may normally be expressed at high levels only during infection by intracellular bacteria. © 1994, Rockefeller University Press., All rights reserved.