Human immunity to alpha(1,3)Galactosyl epitopes (alpha Gal) may provide the means for a successful cancer gene therapy that uses the immune system to identify and to destroy tumor cells expressing the suicide gene alpha(1,3)Galactosyltransferase (alpha GT). Innate antibody specific for cell surface alpha Gal constitutes a high percentage of circulating IgG and IgM immunoglobulins in humans and is the basis for complement-mediated hyperacute xenograft rejection and antibody-dependent cell-mediated cytotoxicity. In humans, the gene for alpha GT is mutated, and cells do not express the alpha Gal moiety. We hypothesized that human tumor cells induced to express the alpha Gal epitope would be killed by the hosts' innate immunity. Previous in vitro work by our group has demonstrated complement-mediated lysis of alpha Gal-transduced human tumor cells in culture by human serum. To induce antibodies to alpha Gal in this in vivo study, alpha GT knockout mice were used to determine whether immunization with alpha Gal could provide protection from challenge with alpha Gal-expressing murine MC38 colon cancer cells. Knockout mice were immunized either a single time, or twice, with rabbit RBC. Antibody titers to alpha Gal measured by indirect ELISA were significantly higher in mice immunized twice and approached the titers observed in human serum. Anti-alpha Gal antibodies were predominantly of the IgG1 and IgG3 subtype. Immunized knockout mice were challenged i.p. with varying doses of alpha Gal(+) MC38 colon carcinoma cells. Nonimmunized control groups consisting of alpha GT knockout mice, and wild-type C57BL/6 mice were challenged as well with MC38 cells. Immunized mice survived and exhibited slower tumor development in comparison to nonimmunized knockout and control mice. This study demonstrates, in vivo, the protective benefit of an immune response to the alpha Gal epitope. Our results provide a basis to pursue additional development of this cancer gene therapy strategy.