Eighty-one patients with a variety of refractory disseminated malignant neoplasms have been treated in the first multiple fixed-dose phase I trial of recombinant leukocyte A interferon (IFL-rA). Each patient received IFL-rA by intramuscular injection, three times weekly for 28 days. Dosages were escalated in different patients from 1 to 136×106 units per injection. The toxic reactions seen with IFL-rA resembled those of nonrecombinant leukocyte interferon and included fever, chills, fatigue, anorexia, myalgia, headache, occasional nausea and vomiting, and dose-dependent reversible leukopenia and hepatic transaminase elevations. The pharmacokinetics of IFL-rA were also comparable with nonrecombinant leukocyte interferon. Objective evidence of antitumor activity was seen in non-Hodgkin's lymphoma, chronic lymphocytic leukemia, Hodgkin's disease, breast cancer, and melanoma, indicating that IFL-rA, the first genetically engineered biological response modifier available for testing in cancer patients, is biologically active in vivo.