Abstract Biomodulators (biologic response modifiers) can affect the tumor, the tumor-bearing host, or both, and thereby help the host to overcome the tumor. Most of the biomodulators have been stimulators of the immune system, while many chemotherapeutic drugs are immunosuppressive. Paradoxic effects of each on the immune response are described. This article briefly describes principles of tumor immunology and gives examples of the various effects of chemotherapy on the immune response. An understanding of how one type of therapy may affect the activity of the other will permit us to design rational combinations at schedules that are additive or synergistic.