Listeria monocytogenes is an intracellular organism that has the unusual ability to live in the cytoplasm of the cell. It is thus a good vector for targeting protein antigens to the cellular arm of the immune response. Here we use a model system, consisting of colon and renal carcinomas that express the influenza virus nucleoprotein and a recombinant L. monocytogenes that secretes this antigen, to test the potential of this organism as a cancer immunotherapeutic agent. We show that this recombinant organism can not only protect mice against lethal challenge with tumour cells that express the antigen, but can also cause regression of established macroscopic tumours in an antigen-specific T-cell-dependent manner.