Tumour-associated monoclonal antibodies (HMFG1, HMFG2 and AUA1) radiolabelled with iodine-131 were given intracavitary (intrapleurally and intrapericardially) to patients with malignant effusions. Ten out of 13 effusions (3 pericardial and 7 pleural) responded completely with no fluid reaccumulation between 3 and 18 months. No clinical or other toxicity was observed. This new method of treatment for recurrent malignant effusions is non-toxic and effective resulting in improved quality of life, and, in some cases, prolongation of survival.