A phase I study to evaluate the use of i.p. infusion of recombinant interleukin-2 (rIL-2) was planned. The following dose levels were calculated: 0.1, 0.3, 1.0, 3.0 and 10 mg/m2/day for 14 days, but only the second levels were reached. In this trial the acute toxic effects at this dosage included cardiac ischemia, transient liver impairment and septic peritonitis. The maximum tolerated dose (MTD) was 0.3 mg/m2/day for 14 days. In addition, two patients developed peritoneal fibrosis. No objective responses were observed. Therefore, in order to explore the biological activity of low (nontoxic) doses, three patients (one untreated and two previously treated with rIL-2) were infused with 0.01 and 0.03 mg/m2/day for 7 days. Potentiation of cytolytic activities in peritoneal lymphocytes and activation of a lymphokine cascade in the ascitic fluid were observed at doses ranging from 0.03 mg/m2/day to 0.3 mg/m2/day. These findings in association with the toxic effects observed at the MTD suggest the use of the minimum effective dose for future locoregional immunotherapeutic protocols.