The major barrier for xenotransplantation in humans is the presence of alpha(1-3) Galactosyl epitopes (alphaGal) in xenogeneic tissue and the vast quantities of natural antibodies (Ab) produced by humans against this epitope. The binding of anti-alphaGal Ab to cells expressing alphaGal triggers a complement-mediated hyperacute rejection of target cells. The hyperacute rejection of whole cancer cells, modified to express alphaGal epitopes, could be exploited as a new cancer vaccine to treat human cancers. We tested this hypothesis in alphaGalactosyltransferase knockout (alphaGT KO) mice which, like humans, do not express alphaGal on their cell surfaces and can produce anti-alphaGal Ab. Forty-five percent of mice with preexisting anti-alphaGal Ab rejected alphaGal positive melanoma cells (B16alphaGal). These mice remained tumor-free for more than 90 days. The majority of control mice injected with B16Null, alphaGal negative cells succumbed to melanoma. The rejection of B16alphaGal induced strong long-lasting antitumor immunity against B16Null measured by the expansion of cytotoxic T lymphocytes. In addition, mice rejecting B16alphaGal were protected against melanoma since they survived a second rechallenge with B16Null. Protected mice developed antitumor immunity in the absence of autoimmune depigmentation (vitiligo). These results show that rejection of alphaGal positive melanoma cells can efficiently boost the immune response to other tumor associated antigens present in alphaGal negative melanoma cells. This study supports the concept of a novel anticancer vaccine to treat human malignancies.