The ATM-2 antibody reacts with rat OX-40 Ligand (OX-40L, CD252), a member of the TNF Ligand superfamily. Although there is controversy regarding mouse OX-40L expression, OX-40L has been shown to be expressed on human dendritic cells and mouse activated B lymphocytes. Rat OX-40L interacts with OX-40 Antigen (CD134) found predominantly on activated T cells. The OX-40L-OX-40 interaction is reciprocally costimulatory in that both T cells and B cells are activated in cross-linking. Rat OX-40L mRNA has been detected in various HTLV-I-transformed T-cell lines: F344-S1, WKA-S1, and ACI-S1. In addition, total rat spleen cells activated with 5 ng/ml PMA and 0.5 µg/ml ionomycin for two days showed OX-40L expression, as determined by flow cytometric analysis, with the ATM-2 mAb. As observed in the mouse and human systems, rat OX-40L also costimulates IL-2 production and proliferation of anti- D3-activated rat T cells. The ATM-2 mAb blocks the costimulatory activity of OX-40L on rat T- cell proliferation.