The purpose of this phase 1 study was to determine the toxicity and effectiveness of recombinant interleukin-2 (RU 49637 Roussel Uclaf-France) administered by continuous pleural infusion for 5 days to patients with different histologic subtypes of pleural cancer. Incremental doses of rIL-2 from 3 x 10(6) to 24 x 10(6) were given via a thin catheter inserted into the homolateral pleural cavity. Patients were evaluated before treatment and 36 days after treatment by computed tomography scan and thoracoscopy with biopsy. Twenty-two patients with malignant pleural effusion (15 malignant pleural mesotheliomas, 6 adenocarcinomas, 1 squamous cell carcinoma) were treated. The maximum tolerated dose (MTD) of rIL-2, defined as the dose that produced grade 3 or greater toxic reactions in 50 percent of the patients, was 24 x 10(6) IU/m2/d. Although some side effects were encountered at any dose, tolerance was acceptable. The main side effect was fluid retention (8 of 22) which never exceeded 10 percent of body weight. Responses were achieved in 10 out of 22 patients with 1 complete remission (mesothelioma) and 9 partial remissions (3 adenocarcinomas and 6 malignant pleural mesotheliomas). Based on these results, we recommend that phase 2 studies using rIL-2 at a dose of 21 x 10(6) IU/m2/d via the intrapleural route be undertaken.