Attempts to treat cancer by stimulation of cell-mediated responses in the host (active immunotherapy) or by adoptive transfer of lymphocytes remain the major areas of research in biologic therapy for cancer. Focus in adoptive immunotherapy has swung decisively from transfer of lymphokine-activated killer cells together with interleukin-2 to transfer of tumor-infiltrating lymphocytes or lymphocytes activated against tumor cells by various procedures and transfected or not with cytokine genes. These treatment approaches are applicable only in major cancer research centers, and they have not yet been proven to be more effective than stimulation of host immune responses with vaccines or cytokines, or both. Immunotherapy with tumor cells transfected with cytokine genes is a promising new approach to active immunotherapy, which is providing new insights into cellular responses to tumors. Increased attention is being given to the role of tumor-derived inhibitors of immune responses, which may limit the effectiveness of cellular therapy. Future treatments may need to block these tumor-derived inhibitors and provide effective cell-mediated responses against tumors.