We have identified two forms of a major histocompatibility complex (MHC) class I molecule, H-2Kb, distinguishable by specific antibodies through a study of a genetically engineered mouse cell line that overexpresses these molecules. One form, a complex associated with beta 2-microglobulin (native, beta 2m+ class I), is detectable by conformation-dependent antibodies. The other form, which remains after preclearing cell lysates of native class I, is only poorly, if at all, associated with beta 2-microglobulin (beta 2m- class I) and is detectable by an antiserum against the cytoplasmic tail region of H-2K molecules. Both forms are also present in normal cell lines. The affinity-purified native class I molecules bind short peptides (8 or 9 residues) and assemble tightly with beta 2-microglobulin. In striking contrast, the beta 2m- class I molecules bind peptides that are longer (> 15 residues) than those bound to native class I molecules. This finding is consistent with the recent evidence that peptides longer than 8-10 amino acid residues are transported into the endoplasmic reticulum and suggests the possibility of a control step for peptide presentation by MHC in which the incompletely processed peptides bind to the heavy chain and a selected fraction undergoes final processing and presentation on the cell surface.