The anti-tumor immunologic effects of plasmid DNA vaccines encoding human carcinoembryonic antigen (CEA) fused to mouse granulocyte/macrophage colony-stimulating factor (GM-CSF) were examined. Immunization of C57BL/6 mice with the CEA-GMCSF fusion plasmids in a three injection, high-dose immunization schedule led to T cell and antibody responses specific for CEA. Mice injected with CEA-GMCSF fusion plasmids also developed IgG autoantibodies to GM-CSF. Tumor challenge with the CEA-expressing syngeneic mouse adenocarcinoma line, MC38-CEA-2, showed delayed tumor growth in mice immunized with the CEA-GMCSF fusion plasmids but complete protection in mice immunized with plasmid encoding CEA alone. In contrast, a single low-dose immunization with CEA-GMCSF fusion plasmids provided better tumor protection than low-dose CEA plasmid alone and resulted in lower titers of GM-CSF antibodies.