The cytokine IL-12 was an early candidate for cancer immunotherapy due to its role in stimulating the release of IFN-γ from T and NK cells to promote tumor clearance. Administering IL-12 therapeutically has been challenging because systemic treatments led to excessive cytokine storm and inflammation. Developing more targeted ways to focus IL-12 activity to the tumor microenvironment is a significant priority. IL-12 is a heterodimer formed by the subunits, IL-12p35 (p35) and IL-12p40 (p40) which are canonically thought to be made together in the same cell. Although p40 can be released as a monomer, it is inert. We have shown that these p40 monomers; however, can combine extracellularly with p35 from stromal cells to form functional IL-12 and help control infection. Based on this newly described pathway, we hypothesized that treatment with recombinant p40 monomers would allow it to combine with tumor-released p35 and focus IL-12 formation to the local microenvironment. This would facilitate targeted tumor clearance while limiting systemic immunopathology. To test this hypothesis, we evaluated treatment with p40 monomers in mice challenged with a subcutaneous tumor (MB49). We found that monomeric 40 itself was inert – neither capable of inducing IFN-γ or inhibiting the activity of heterodimeric IL12. Nevertheless, recombinant p40 monomers helped control tumor growth in p40-deficient mice. This suggests that p40 can combine with p35 released in the tumor microenvironment to form functional IL-12. Our data provide evidence for an alternate pathway of IL-12 formation in vivo and suggest that this can be exploited therapeutically to avoid systemic inflammation.