Peptide fragments are displayed on the APC surface by MHC class I molecules as ligands for appropriate TCR. Sequence analysis of MHC-bound peptide mixtures has suggested that naturally processed peptides are defined by their length and by the presence of MHC allele-defined consensus motifs. To define the minimal OVA peptide presented by Kb MHC, and the requirements for generation of endogenous OVA/Kb complex, APC were transfected with OVA cDNA constructs. We show that optimal stimulation of OVA/Kb-specific B3Z T cell hybrid by Kb-APC requires the OVA257-264 peptide (SIINFEKL, SL8) whether added exogenously, or when synthesized endogenously as precursor polypeptides. Thus, all information necessary for expression of the OVA/Kb complex is contained within the Kb octapeptide motif shared by SL8. Unexpectedly, the SL8/Kb or the influenza NP/Db complexes were also generated in APC even when the peptide coding sequences were placed in incorrect translational reading frames. By contrast, identical manipulations of the translational reading frame of the lacZ reporter gene reduced protein synthesis to undetectable levels demonstrating the remarkable efficiency with which endogenous peptide/MHC are generated in and presented by APC to T cells. These characteristics of the endogenous presentation may explain how large numbers of distinct peptide/MHC complexes are displayed on the cell surface and surveyed by TCR.