An agonistic antibody that recognizes the co-stimulatory receptor OX40 (CD134;TNFRSF4), with potential immunostimulatory activity. Upon administration, anti-OX40 antibody PF-04518600 selectively binds to and activates OX40; which induces proliferation of memory and effector T-lymphocytes. In the presence of tumor-associated antigens (TAAs), this may promote a T-cell-mediated immune response against TAA-expressing tumor cells. OX40, a cell surface glycoprotein and member of the tumor necrosis factor receptor superfamily (TNFRSF), is expressed on T-lymphocytes and plays an essential role in T-cell activation.