A plethora of data from many groups supports the existence of a cancer immunosurveillance process in mice. Support comes from the use of gene-targeted mice with congenital immunodeficiencies, as well as using adult wild type mice rendered immunodeficient by injection of neutralizing or depleting antibodies. Together this work shows that loss of either innate or adaptive immunity leads to increased carcinogen-induced and spontaneous tumors in the immunodeficient mice [adapted from (2)].