The usefulness of the cell-wall skeleton of Mycobacterium bovis BCG (BCG-CWS) to the management of malignant pleural effusion is demonstrated in this report. In two patients with malignant pleurisy the oil-attached BCG-CWS reduced the pleural effusion in volume when injected intrapleurally. The following intradermal injections of BCG-CWS into the bilateral upper arms repeated monthly prevented the reaccumulation of the effusion and improved other clinical conditions of the patients. The possible mechanism responsible for this effect is discussed in relation to previous experimental and clinical studies.