The immune system can deal effectively with the majority of viruses and bacteria, less effectively with parasites, and very poorly with cancer. Why is this so? Why are McFarlane Burnet's and Lewis Thomas' predictions that the immune system is in- volved in ridding the body of cancer cells, encapsulated in the catchy phrase "immunologic surveillance," so difficult to experi- mentally establish? Cancer differs from infectious agents in being derived from the host. Hence, it has been postulated that cancer cells lack anti- gens that the immune system can recognize. They are not "im- munogenic. "...