Interferon production in vitro was induced by poly I: C and PHA in leukocytes obtained from patients with cancer, benign diseases and control subjects. The interferon response per lymphocyte was relatively constant in all groups. However, interferon production was slightly inhibited in patients receiving cancer chemotherapy. The peak response of interferon occurred at 48 hours after the initiation of the combined culture in each specimen. From our observations, it was suggested that interferon production in vitro seems to be useful marker for evaluation of effect of chemotherapy and or immunotherapy, and, furthermore, in the treatment of cancer patients with interferon inducers, there seems to be optimum BRM dose and optimum timing of administration.