Cancer may be thought of as an immunological disorder that arises because certain 'transformed' cells, endowed with the propensity to divide, have learned to evade detection by the immune system. The prospect of intervention by 'immunotherapy' depends very much on our ability to either  render cancer cells more recognizable to the immune system, or  potentiate the immune system towards a more effective recognition of cancer cells. There is now direct evidence that suppression of the major histocompatibility complex class I antigens, a family of cell-surface glycoproteins required for the presentation of cancer cells to the immune system, is directly responsible for the ability of tumor cells to escape immune surveillance. It has been shown that cancer cells can be made immunogenic either by the expression of an exogenous class I gene introduced by DNA-mediated gene transfer, or by the derepression of endogenous class I genes with interferon; these cells are efficiently rejected by the immune system. Even more interesting is the finding that the immune system can be potentiated to reject tumors by immunization with homologous tumor cells that have been manipulated to express normal levels of class I antigens. Since increasing numbers of human tumors have been found to have greatly reduced levels of class I antigens, these findings suggest a direct route to immunotherapy that involves debulking of the tumor mass, raising the level of class I antigens in a small number of explanted tumor cells, and re-immunizing the host.