Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) has been known to induce tumor-specific apoptosis and to share the structural and functional char-acteristics with the proteins of TNF family. Recently, the crystal structure of humanTRAIL showed that TRAIL is a homotrimeric protein whose subunits contain mainly b-sheets. We characterized the structural changes of recombinant human TRAIL induced byacidification and the biological implication of the structural characteristics at acidic pH in the interaction with the lipid bilayer. .