A phase-I study of the recombinant, non-mutagenized interleukin 2 (rlL2, Bioleukin'") was performed in 12 melanoma patients (Pts). From 100 to 800 pg/m2 of rlL2 were administered by i.v. bolus injection, TID for 4-8 days. Side-effects included fever, malaise, low serum K + and C a + + values, electrocardiographic abnormalities, leukopenia and thrombocytopenia. No major organ toxicity and no significant fluid retention were observed at the administered doses. Treatment induced a rapid depletion of peripheral blood lymphocytes (PBL) with a rebound (2-6 times the pre-treatment values), 24-48 hr after rlL2 discontinuation. PBL obtained between the 5th treatment day and the 2nd post-treatment day showed: (a) enhanced proliferation (I 1/12 Pts) with stimulation indexes of 6-52; (b) increased cytotoxicity against autologous tumor cells (212 Pts), allogeneic melanomas (517 Pts), the Daudi (5/6 Pts) and K562 cell lines (7112 Pts); and (c) increased expression of IL2 receptors (8112 Pts) and of DR antigens (6/12 Pts). Lymphocytes collected 1-2 days after treatment and activated in vitro with rlL2 showed a more rapid development of tumor cytotoxicity, with an earlier loss of activity. Spontaneous proliferation, autologous or allogeneic tumor cytotoxicity and expression of IL2 receptors obtained after in vivo treatment with rlL2 were significantly weaker than those induced during in vitro stimulation. No major objective responses were detected in these patients.