Lymphocytes isolated from malignant effusion were induced to become LAK cells after in vitro culture with rIL-2. 28 patients with malignant effusion were treated by i.p. or intrapleural administration of autologous LAK cells combined with rIL-2 or by rIL-2 alone. The effusion disappeared in 13 patients and significantly decreased in another 13. Two patients did not respond to the treatment. Tumor cells in effusion disappeared or significantly decreased and lymphocytes significantly increased in all responses. Except for transient fever in 9 patients, no serious side effects were found.