PURPOSE: Whilst immune checkpoint inhibitors such as anti-PD-L1 are rapidly becoming the standard of care in the treatment of many cancers, only a subset of treated patients have long-term responses. Interleukin 12 (IL-12) promotes anti-tumor immunity in mouse models, however systemic recombinant IL-12 had significant toxicity and limited efficacy in early clinical trials. EXPERIMENTAL DESIGN: We therefore designed a novel intratumoral (IT) IL-12 mRNA therapy to promote local IL-12 tumor production whilst mitigating systemic effects. RESULTS: A single IT dose of mouse (m)IL-12 mRNA induced IFN_ and CD8+ T cell-dependent tumor regression in multiple syngeneic mouse models, and animals with a complete response demonstrated immunity to re-challenge. Anti-tumor activity of mIL-12 mRNA did not require NK and NKT cells. mIL-12 mRNA anti-tumor activity correlated with TH1 tumor microenvironment (TME) transformation. In a PD-L1 blockade monotherapy-resistant model, anti-tumor immunity induced by mIL-12 mRNA was enhanced by anti-PD-L1. mIL-12 mRNA also drove regression of un-injected distal lesions, and anti-PD-L1 potentiated this response. Importantly, IT delivery of mRNA encoding membrane-tethered mIL-12 also drove rejection of un-injected lesions with very limited circulating IL-12p70, supporting the hypothesis that local IL-12 could induce a systemic anti-tumor immune response against distal lesions. Furthermore, in ex vivo patient tumor slice cultures, human IL-12 mRNA (MEDI1191) induced dose-dependent IL-12 production, downstream IFN_ expression and TH1 gene expression. CONCLUSIONS: These data demonstrate the potential for intratumorally delivered IL-12 mRNA to promote TH1 TME transformation and robust anti-tumor immunity.