The depletion of tumor stromal cells that are marked by their expression of the membrane protein fibroblast activation protein-α (FAP) overcomes immune suppression and allows an anticancer cell immune response to control tumor growth. In subcutaneous tumors established with immunogenic Lewis lung carcinoma cells expressing ovalbumin (LL2/OVA), the FAP⁺ population is comprised of CD45⁺ and CD45⁻ cells. In the present study, we further characterize the tumoral FAP⁺/CD45⁺ population as a minor subpopulation of F4/80^hi/CCR2⁺/CD206⁺ M2 macrophages. Using bone marrow chimeric mice in which the primate diphtheria toxin receptor is restricted either to the FAP⁺/CD45⁺ or to the FAP⁺/CD45⁻ subset, we demonstrate by conditionally depleting each subset that both independently contribute to the immune-suppressive tumor microenvironment. A basis for the function of the FAP⁺/CD45⁺ subset is shown to be the immune inhibitory enzyme, heme oxygenase-1 (HO-1). The FAP⁺/CD45⁺ cells are the major tumoral source of HO-1, and an inhibitor of HO-1, Sn mesoporphyrin, causes the same extent of immune-dependent arrest of LL2/OVA tumor growth as does the depletion of these cells. Because this observation of immune suppression by HO-1 expressed by the FAP⁺/CD45⁺ stromal cell is replicated in a transplanted model of pancreatic ductal adenocarcinoma, we conclude that pharmacologically targeting this enzyme may improve cancer immunotherapy. Cancer Immunol Res; 2(2); 121–6. ©2013 AACR.