Thirty-three patients with advanced malignancy were treated with Wellferon. Doses ranging from 0.75 X 10(6) to 50 X 10(6) U were administered intramuscularly every 12 h for a 7-day course of therapy. Courses were repeated every 4 weeks as a function of tumor response. Toxicity resulted in fever, chills, malaise, leukopenia, thrombocytopenia, nausea and/or vomiting, diarrhea, hepatocellular damage, and, in a single case, gastrointestinal bleeding (which was a possible cause of patient death). Toxicity tended to increase with increasing dose, and 30 X 10(6) units every 12 h for 7 days was considered to be the maximally tolerated dose. Partial responses were seen in three patients with diagnoses of renal cell carcinoma, diffuse histocytic lymphoma, and Hodgkin's disease. Minimal responses were seen in four patients with diagnoses of chronic lymphocytic leukemia, multiple myeloma (two patients), and breast cancer. Positive response to therapy did not correlate with dose level.