Purified anti-mouse CD134 monoclonal antibody detects the mouse OX-40 antigen (CD134) which is a member of the tumor necrosis factor (TNF) receptor/nerve growth factor (NGF) receptor superfamily. The OX-40 antigen is the sole receptor for the OX-40 ligand. This antibody (clone OX-86) labels the OX-40 antigen found on both mouse CD4 and CD8 single positive activated T cells. This is in contrast to rat OX-40 which is only found on CD4 positive T blasts (not CD8). Also, the OX-86 mAb does not block binding of the OX-40L. This antibody is suitable for use in flow cytometry.