Interleukin (IL) 2 plays an important role in enhancing the immune response, whereas IL-4 has pluripotent activities which include affecting immune function. Preclinical data suggest that the combination might have enhanced immunomodulatory activity. In this Phase I trial in patients with advanced solid tumors, both IL-2 and IL-4 were given by separate s.c. injections simultaneously daily, 5 days in a row, Monday through Friday, for 3 consecutive weeks, followed by a 1-week break from treatment. Cycles could be repeated. The dose of IL-2 was kept constant at 9 x 10(6) IU/m2/injection while the dose of IL-4 was escalated beginning at 100 microgram/m2/injection and increasing by 100-microgram/m2 increments to a planned level of 400 microgram/m2/injection. Sixteen patients were entered in this study, with one patient being ineligible because of the presence of brain metastases. Of the 15 eligible patients, there were 14 males and 1 female, with a median age of 54 (range, 38-67) years and initial performance status of 0 in 5 patients and 1 in 10 patients. Patients were treated at levels of up to 300 microgram/m2/injection of IL-4 before the study was closed due to withdrawal of the drug by the manufacturer. The most commonly observed toxicities were fatigue, fever and chills, local reaction, nausea/vomiting and anorexia, headache and nasal stuffiness, and coughing, sometimes with the production of clear white sputum, more common in smokers. Duodenal ulcers occurred in one patient and one patient had grade 4 cardiac toxicity consisting of an asymptomatic minimal elevation of the creatinine phosphokinase MB isoenzyme (CPK-MB). Grade 3 hyponatremia occurred in two patients, and elevated liver function tests and creatinine occurred but were not dose limiting. Eosinophilia of unknown significance occurred in all patients. There were statistically significant elevations in absolute numbers of most T-cell subsets examined, without changes in circulating B cells. No antibodies to the IL-4 were found after one cycle. One patient with renal cell carcinoma showed a significant decrease in tumor burden after one cycle of treatment. Because of the IL-4 withdrawal, the maximum tolerated dose for this combination of drugs given by the route and schedule used here was not determined and will require additional testing. Subcutaneous IL-2 and IL-4 given simultaneously show important immunomodulatory and antitumor effects and should be tested further in cancer patients.