Recombinant interleukin (IL)-4, 5 μg/kg thrice weekly for 3 weeks followed by a 2-week rest period (1 cycle) was administered to 32 eligible previously treated B-cell chronic lymphocytic leukemia (7 patients) or low-grade B-cell lymphoma patients (25 patients). Two cycles were given before response was evaluated. IL-6 serum levels were evaluated before therapy in all patients and at 12 weeks on study in 7 patients. None of the chronic lymphocytic leukemia patients responded. A partial response was observed in 3 lymphoma patients of 1.2, 3.0, and 3.5 months' duration and stable disease (median 1.5 mo) was observed in another 7 lymphoma patients. The median survival from registration on study was 29.7 months with 7 patients alive at the time of analysis for a median follow-up of 72.8 months. Toxicity was generally mild with no grade 4 nonhematologic toxicity observed. Recombinant IL-4 treatment was well tolerated in this study but had minimal antitumor activity.