The treatment of cancer by harnessing immune responses has long been pursued. Efforts to turn on the immune system against cancers with inactivated tumor vaccines or intratumor injections of bacterial products to induce local inflammation and recruit an antitumor immune response have led to anecdotal successes. Increasing knowledge about how the immune system is activated, coupled with advances in recombinant DNA technology, has allowed the clinical testing of immune-stimulating cytokines such as interferons and interleukins. These trials have led to a low frequency of durable tumor responses in selected cancers such as melanoma and renal-cell carcinoma at the expense of . . .