In a group of 14 patients with a malignant pleural effusion treated by intrapleural administration of Corynebacterium parvum, in addition to other treatment of the generalized malignant process the activity of Nk cells was investigated before administration, on the 7th and 21st day after instillation of the vaccine. The mean rise of this activity was significant at the 5% level of significance. The activity of Nk cells was equally significantly influenced by age. The survival of patients, although longer in patients who responded positively, was not significantly correlated with the increased activity of Nk cells. Its increase was observed more frequently in the group of patients given the English preparation Coparvax. In the discussion some aspects of the relationship of Nk activity and anti-tumourous immunity are discussed.