We conducted a phase I study of low-dose cyclophosphamide and recombinant interleukin-2 (rIL-2) in 66 patients with advanced cancer resistant to standard therapy. All patients were evaluable for toxicity and 46 patients were evaluable for antitumor response. Patients evaluable for antitumor response included 23 with malignant melanoma, 10 with renal cell carcinoma, 4 with colon cancer, and 9 with various other solid tumors. All patients received i.v. cyclophosphamide (350 mg/m2) on day 1 followed by rIL-2 via 15 min i.v. infusion on days 4-8 and 11-15. The doses of rIL-2 ranged from 6.0 to 36.0 x 10(6) IU/m2. Each treatment cycle consisted of 21 days and a total of 113 cycles was administered. The number of treatment cycles administered per patient ranged from 1 to 8. The dose-limiting toxicities associated with rIL-2 included altered mental status, arthralgias, diarrhea, fatigue, fever, hypotension, nausea/vomiting, and peripheral edema. Twelve patients (18%) were removed from the study secondary to toxicity. Among the evaluable patients, 2 (4%) (malignant melanoma, renal cell carcinoma) developed a partial remission, 13 (29%) maintained stable disease, and 31 (67%) developed progressive disease. We conclude that the combination of low-dose cyclophosphamide and rIL-2 is tolerable in most patients but our data do not suggest an improved response rate for the combination vs. rIL-2 alone.