Abstract: Natural killer activity was investigated in 30 patients with malignant lymphoma and in 20 healthy adults. The mean activity was significantly lower in untreated patients and. in patients in the active phase of the disease during treatment, as compared with the controls. There was a similar mean value of NK activity in patients in complete remission, as compared with normal subjects. NK activity correlated with the response to therapy. The results suggest that the study of NK function might offer a useful tool in following clinical course and the efficacy of therapy in patients with malignant lymphoma.