Poor uptake by antigen-presenting cells (APC) is a major reason for low immunogenicity of autologous tumor vaccines. This immunogenicity may be increased by exploiting the natural anti-Gal antibody that is present in humans as approximately 1% of circulating IgG. Anti-Gal binds to alpha-gal epitopes (Galalpha1-3Galbeta1-4GlcNAc-R) on vaccinating tumor cells and opsonizes them for effective uptake by APC. This epitope is synthesized in human tumor cells by transduction with AdalphaGT- a replication deficient adenovirus containing the alpha1,3galactosyltransferase (alpha1,3GT) gene. Protection against tumors by immunization with AdalphaGT-transduced tumor cells was studied in alpha1,3GT knockout (KO) mice, challenged with the highly tumorigenic BL6 melanoma cells. These mice lack alpha-gal epitopes and can produce anti-Gal. Immunization of KO mice with AdalphaGT-transduced BL6 cells protects many of the mice against challenge with live BL6 cells lacking alpha-gal epitopes. Immunization with AdalphaGT transduced autologous tumor cells may serve as adjuvant immunotherapy delivered after completion of standard therapy. This method may complement another gene therapy method in which GM-CSF-secreting vaccinating tumor cells recruit APC to vaccination sites. Anti-Gal-opsonized vaccinating tumor cells will be effectively internalized by GM-CSF recruited APC and transported to draining lymph nodes for processing and presentation of tumor antigens. Alternatively, injection of AdalphaGT directly into solid tumor masses of cancer patients may result in anti-Gal-mediated destruction of the transduced tumor cells in a manner similar to xenograft rejection. The subsequent uptake of anti-Gal-opsonized tumor membranes by APC results in their effective transportation to lymph nodes where processed tumor antigens may elicit a protective antitumor immune response.