0x40, a member of the tumor necrosis factor (TNF) receptodnerve growth factor (NGF) receptor superfamily was first identified as a marker of activated rat CD4'cells with the MRC OX40 monoclonal antibody (mAb). A ligand for OX40 (called OX40 ligand or OX40L) has recently been identified and has sequence similarity to TNF. Mouse OX40L-immunoglobulin fusion protein (OX4OL-Ig) binds to activated mouse CD4'and CD8'cells (Baum, PR et al., EMBO J. 1994.13: 3992) suggesting that OX40 could have a differential pattern of expression on mouse and rat T cells. This, however, did not rule out the presence of an alternative receptor on CD8'cells that also binds the OX40L. We have compared the binding of the MRC OX40 mAb with that of OX4OL-Ig to activated rat lymph node cells and show that both recognize the same protein, namely OX40 which is expressed on CD4'and CD4'CD8a'cells, but not on CD4-CD8'cells. We have raised a new mAb (MRC 0x86) using recombinant mouse OX40 protein and show by two-color flow cytometry that mouse OX40 is expressed on CD4 and CD8 single-positive cells. In addition, the new MRC OX86 mAb, unlike the MRC OX40 mAb, did not block binding of the OX40L. We conclude that OX40 is differentially expressed on activated mouse and rat T cells and is the sole receptor for the OX40L.