Immunometabolism is emerging as a critical determinant of cancer pathophysiology. A balance between tumor promotion and elimination is dependent on the state of functional polarization of macrophage populations within the tumor microenvironment. Lactic acid generated by lactate dehydrogenase-A is a key metabolite that facilities the immunosuppressive tumor microenvironment. Deletion of lactate dehydrogenase-A in myeloid cells restores the immunocompetent tumor microenvironment by reversing macrophage phenotype and antitumor immunity. Immunofluorescence staining revealed an increased number of infiltrating inducible nitric oxide synthase-positive (red) and M1-skewed F4.80-positive (green) macrophages in K-Ras tumors after deletion of lactate dehydrogenase-A. For details, see article by Seth and colleagues on page 3632.