Twenty-three cancer patients with pleural and/or abdominal effusion refractory to chemotherapy were treated with intracavitary beta-interferon after drainage. The overall response rate at 30 days was 35% for the whole group of patients, 25% for those with pleural effusion, and 40% for those with ascites. In 15 patients the mean time to re-accumulation of fluid after drainage only was 8.7 days, while it approached 30.3 days after intracavitary beta-interferon. This difference was statistically significant (P less than 0.001). The treatment was well tolerated with only mild and transient pain in 3 cases.