Both bleomycin and tetracycline have been suggested as the sclerosing agent of choice in the management of malignant pleural effusions. To determine if one drug is superior to the other in this role, patients with malignant pleural effusions were randomly assigned to receive either bleomycin or tetracycline in the previously evacuated pleural space through a thoracostomy tube. Following instillation of the assigned agent, the tube was clamped for 8 hours and then reattached to suction. When the chest tube drainage had slowed to less than 40 ml in a 24-hour period or if 7 days had passed, the tube was removed. Pleural sclerosis was attempted 42 times in 34 patients. No statistically significant differences were found between the two treatment groups when prevention of effusion reaccumulation and time to removal of the chest tube (efficiency) were compared. Side effects including pleural pain and fever, occurred with both agents, but were manageable. Since one drug was not clearly superior to the other, and bleomycin is more costly, we suggest that tetracycline rather than bleomycin be used when pleural sclerosis is needed to manage malignant pleural effusions.