Twenty-one patients with proven recurrent malignant pleural effusions were randomly allocated to treatment groups receiving either intrapleural Corynebacterium parvum in a dose of 7 mg or intrapleural mustine (20 mg). The designated intrapleural therapy was repeated on one occasion if further pleural aspiration was required. Corynebacterium parvum (nine patients) proved superior to mustine (12 patients) in suppressing the reaccumulation of pleural fluid, and was associated with only minimal side-effects of fever and nausea in two patients. Mustine caused marked nausea and vomiting in almost all patients. Three of the four patients who were deemed "failures" after mustine therapy had complete suppression of pleural fluid reaccumulation after a single dose of C parvum, the survival of the fourth being too short to assess a response adequately. There were no failures in the C parvum treated group. Corynebacterium parvum appears to be an effective, well-tolerated agent in the management of recurrent pleural effusions. The relative contribution of its potent immunological stimulant effect to its mode of action remains uncertain.