IL-12 can enhance the development of effective immune responses against tumors as well as against certain infectious agents. It is therefore a potential candidate for therapeutic use in cancer therapy and in the design of vaccines against several infectious diseases. Several studies have demonstrated that IL-12 could efficiently induce tumor regression in animal models. To investigate the antitumor effect of direct gene transfer of human IL-12 into tumors, human IL-12 p35 and p40 cDNAs were cloned into the Semliki Forest virus (SFV) vector pSFV1. In order to express the two subunits from the same vector, the p35 and the p40 cDNAs were cloned into pSFV1, each under the control of a subgenomic SFV promoter. Recombinant RNA produced by in vitro transcription of SFV-IL-12 construct, was packaged into SFV viral particles with the use of a non-packageable helper RNA. We show that human tumor cell lines infected in vitro in vivo with recombinant SFV-IL-12 viral particles secrete high levels of biologically active heterodimeric p35/p40 IL-12, as demonstrated using ELISA and biological assays.