MAJOR histocompatibility complex (MHC) class I molecules present peptides derived from cellular proteins to cytotoxic T lymphocytes (CTLs)^1–5, which check these peptides for abnormal features. How such peptides arise in the cell is not known. Here we show that the MHC molecules themselves are substantially involved in determining which peptides occur intracellularly: normal mouse spleen cells identical at all genes but MHC class I express different patterns of peptides derived from cellular non-MHC proteins. We suggest several models to explain this influence of MHC class I molecules on cellular peptide composition.