Abstract More than half of patients with solid tumor malignancies undergo treatment with radiotherapy (RT). In addition to causing direct tumor cell death, RT results in release of tumor antigen and damage associated molecular patterns that elicit a CD8 T cell and IFN-g dependent anti-tumor immune response. To further bolster this response, RT has been combined with immune checkpoint inhibitors such as anti-PD-1 and anti-CTLA-4. Despite a modest increase in survival when these therapies are used alone or together in conjunction with RT, many patients fail to respond to treatment, suggesting other mechanisms of immune suppression exist in the tumor microenvironment (TME). Using syngeneic B16 F10 melanoma and C38 colorectal adenocarcinoma models, we have studied the role of the inhibitory receptor NKG2A. We have observed that NKG2A is expressed only on tumor infiltrating lymphocytes (TILs) and expression of this receptor does not change with RT. Furthermore, RT increases intratumoral expression of the ligand for NKG2A, Qa-1b, as does IFN-g stimulation in vitro. Blockade of NKG2A alone through use of B16 F10 cells lacking Qa-1b expression or blocking antibodies did not significantly increase survival of mice treated with RT. Further analysis revealed that among CD8 TILs, only a minority of cells express NKG2A alone, with most TILs co-expressing NKG2A and PD-1 or PD-1 alone. Thus, we combined anti-NKG2A and anti-PD-1 blockade and observed increased survival in mice treated with RT, whereas either therapy alone was ineffective. These results suggest that NKG2A blockade could be combined with RT and existing immunotherapies clinically to improve patient response. Supported by awards T32AI007285 from the NIAID and R01CA028332 from the NCI.