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The human OX-40 cell surface antigen is a CD4+ T cell activation marker that acts as a costimulatory receptor and is a member of the nerve growth factor receptor/tumor necrosis factor (TNF) receptor family. Using a soluble form of the receptor, the extracellular region fused with human immunoglobulin Fc, we expression cloned the human OX-40 ligand cDNA from a library derived from an activated B lymphoblastoid cell line MSAB. The encoded protein is identified as gp34, a type II transmembrane antigen previously known to be expressed only by human T cell lymphotropic virus 1-infected cells. We describe gp34 as a new member of the TNF family, and find that the recombinant ligand expressed in COS cells costimulates phorbol myristate acetate, phytohemagglutinin, and anti-CD3-induced CD4+ T cell proliferation.