CLASS I major histocompatibility complex (MHC) molecules present antigens that are produced within the presenting cell or penetrate from the vacuolar system into the cytosol for processing. Most studies of exogenous antigen processing have used soluble antigens, which are not efficiently presented by class I MHC molecules1 and do not elicit CDS T-cell responses in vivo. But particulate antigen preparations with no known mechanism for cytosolic penetration can also elicit CDS T-cell responses in vivo2-7. We report here that phagocytosis of bacteria with no mechanism for cytosolic penetration also results in presentation of bacterial antigens by class I MHC molecules. Moreover, this mechanism is resistant to cycloheximide and Brefeldin A, which block the classical class I processing pathway. These results suggest a novel vacuolar class I processing pathway for exogenous phagocytic antigens.