An anti-human glucocorticoid-induced tumor necrosis factor receptor (GITR) agonistic monoclonal antibody (MoAb) with potential immunomodulating activity. Anti-GITR monoclonal antibody MK-4166 binds to and activates GITRs found on multiple types of T-cells. This stimulates the immune system and induces both the activation and proliferation of tumor-antigen-specific T effector cells, and suppresses the function of activated T regulatory cells. This leads to tumor cell eradication. Also, this agent is shown to act synergistically with chemotherapeutic drugs in multiple cancer models. GITR, a member of the TNF receptor superfamily, is expressed on the surface of multiple types of immune cells, including regulatory T-cells, effector T-cells, B-cells, and natural killer (NK) cells.