The OX-40 receptor (OX-40R) is a cell surface glycoprotein of the tumor necrosis factor receptor family that is expressed primarily on activated CD4 T cells. Engagement of OX-40R by the OX-40 ligand (OX-40L) is known to costimulate the production of cytokines by activated T lymphocytes and to rescue effector T cells from activation-induced cell death. It was previously reported that in vivo ligation of OX-40R by administration of OX-40L:immunoglobulin fusion protein or OX-40R monoclonal antibody (mAb) resulted in a significant prolongation of survival of tumor-bearing mice in four histologically distinct solid tumors. In this study, we demonstrate that the therapeutic efficacy of OX-40R mAb was influenced by the tumor burden, the intrinsic immunogenicity of the tumor as well as by the histological site of tumor growth. Whereas subdermal and intracranial growth of weakly immunogenic MCA 203 and MCA 205 sarcomas and GL261 glioma were susceptible to the mAb treatment, established pulmonary MCA 205 metastases were refractory to the same regimen of treatment. Furthermore, the mAb administration had no impact on the growth of the poorly immunogenic B16/D5 mela noma. Tumor regression mediated by OX-40R mAb was dependent on the participation of both CD4 and CD8 T cells and as a result of tumor rejection, a long-term tumor-specific immunity was established. Analysis of tumor-infiltrating T cells revealed the presence of a far greater number of OX-40R+ T cells of both CD4 and CD8 phenotypes in the intracranial immunogenic GL261 glioma than that in the poorly immunogenic B16/D5 melanoma. These results suggest that ligation of OX-40R on activated T cells in situ in the tumor may provide a necessary costimulatory signal to augment immune responses leading to tumor regression and immunological memory.