Cytokines have recently appeared to be effective in the palliative therapy of neoplastic effusions. The present study was carried out to evaluate the efficacy and the tolerability of an intracavitary injection of IL-2 in patients with neoplastic effusion due to solid tumors. The study included 14 patients with cytologically positive effusion (pleura, 11; peritoneum, 2; pericardium, 1). Tumor histotypes were: mesothelioma, 5; non-small cell lung cancer, 3; breast cancer, 2; ovarian cancer, 2; cervix carcinoma, 1; unknown primary tumor, 1. The efficacy was evaluated according to the criteria of Paladine et al. (Cancer 38: 1903, 1976). An objective response was achieved in 10/14 (71%) patients (4 CR, 6 PR), with a median duration of 4 months (range, 2-8). No important toxicity was seen. This preliminary study showed that low dose IL-2 given intracavitarily is an effective and well-tolerated therapy in patients with neoplastic effusions.