Tumor necrosis factor (TNF) and lymphotoxin-α (LT-α) are members of a family of secreted and cell surface cytokines that participate in the regulation of immune and inflammatory responses. The cell surface form of LT-α is assembled during biosynthesis as a heteromeric complex with lymphotoxin-β (LT-β), a type II transmembrane protein that is another member of the TNF ligand family. Secreted LT-α is a homotrimer that binds to distinct TNF receptors of 60 and 80 kilodaltons; however, these receptors do not recognize the major cell surface LT-α-LT-β complex. A receptor specific for human LT-β was identified, which suggests that cell surface LT may have functions that are distinct from those of secreted LT-α.