CD154, formerly known as CD40 ligand and gp39, is a type II integral membrane protein and a member of the tumor necrosis factor (TNF) family of ligands. It is an important accessory molecule in T cell-B cell costimulatory interactions and is expressed predominantly on activated CD4+ T lymphocytes. It is also present on the surface of activated Th0, Th1, and Th2 T cell clones. Its expression is transient and cyclosporin-sensitive. The MR1 monoclonal antibody binds to murine CD154 with high affinity, blocks binding to CD40, and blocks CD154 function. Administration of this antibody to mice blocks the ability to mount primary and secondary immune responses to TD antigens yet does not alter the immune response to TI antigens.