A total of 745 serum interferon (IFN) determinations were carried out on the serum of 332 cancer patients with a variety of tumors. Disease activity was defined as either 'active' or 'nonactive' as based on clinical examination, the results of laboratory tests and radiological criteria. IFN levels were estimated for both disease activity and mode of treatment. A statistically significant higher level of IFN in the serum of active, as compared to that of nonactive disease, was established. Furthermore, a slight increase in the IFN level was observed in patients before initiating chemotherapy as compared to untreated patients, followed by a decrease in those patients undergoing chemotherapy. These differences between groups were not found to be statistically significant. However, in the cases of rectum, prostate and female genital cancers, a significant elevated level of serum IFN was observed at a late stage of the disease, followed by a decrease after chemotherapy treatment. In addition, a significant lower level of serum IFN was detected in breast cancer patients shortly after radiotherapy as compared to those before radiotherapy. We conclude that IFN, known to play a role in the control mechanisms of developing malignant processes, can be determined in the serum and used to monitor disease activity in individual patients. The inhibition of IFN release depends in part on therapy and can be used for monitoring treatment schedules along the course of disease.