Interleukin (IL)-2 is a pharmacologically important cytokine secreted by T-lymphocytes. Recombinant IL-2 (rIL-2) has been modified and produced in many systems. Mass production of rIL-2 is the prerequisite for its wide application. Using a site-directed mutagenesis strategy, we first generated a gene coding for a new type of mutant of human IL-2 (MhIL-2), in which we replaced the cysteine-125 in human IL-2 with alanine, the leucine-18 with me thionine, and the leucine-19 with serine. Then we investigated the possibility of its production of MhIL-2 in a Pichia pastoris system. High-level secreted expression of MhIL-2 was achieved by methanol induction. When purified with ultra filtration, cation-exchange chromatography, and Sephadex G100 gel filtration, about 100 mg of MhIL-2 with high purity was obtained from I L of ferment supernatant. Biologic activity assay revealed that the purified recombinant protein displayed increased activity on proliferation of IL-2-dependent CTLL-2cells. These results suggest that MhIL-2 is an improved IL-2 mutant that might hold great promise for clinical use, and that P. pastoris is an excellent system for the mass production of biologically active hIL-2.