Previous studies suggest that the core protein of hepatitis C virus (HCV) has a pleiotropic function in the replication cycle of the virus. To understand the role of this protein in HCV pathogenesis, we used a yeast two- hybrid protein interaction cloning system to search for cellular proteins physically interacting with the HCV core protein. One such cellular gene was isolated and characterized as the gene encoding the lymphotoxin-β receptor (LT-βR). In vitro binding analysis demonstrated that the HCV core protein binds to the C-terminal 98 amino acids within the intracellular domain of the LT-βR that is involved in signal transduction, although the binding affinity of the full-length HCV core protein was weaker than that of its C-terminally truncated form. Our results also indicated that the N-terminal 40-amino-acid segment of the HCV core protein was sufficient for interaction with LT-βR and that the core protein could form complexes with the oligomeric form of the intracellular domain of LT-βR, which is a prerequisite for downstream signaling of this receptor. Similar to other members of the tumor necrosis factor (TNF) receptor superfamily, LT-βR is involved in the cytotoxic effect of the signaling pathway, and thus we have elucidated the biological consequence of interaction between the HCV core protein and LT-βR. Our results indicated that in the presence of the synergizing agent gamma interferon, the HCV core protein enhances the cytotoxic effects of recombinant forms of LT-βR ligand in HeLa cells but not in hepatoma cells. Furthermore, this enhancement of the cytolytic activity was cytokine specific, since in the presence of cycloheximide, the expression of the HCV core protein did not elicit an increase in the cytolytic activity of TNF in both HeLa and hepatoma cells. In summary, the HCV core protein can associate with LT-βR, and this protein-protein interaction has a modulatory effect on the signaling pathway of LT-βR in certain cell types. Given the known roles of LT-βR/LT-α1β2 receptor-ligand interactions in the normal development of peripheral lymphoid organs and in triggering cytolytic activity and NF- κB activation in certain cell types, our finding implies that the HCV core protein may aggravate these biological functions of LT-βR, resulting in pathogenesis in HCV-infected cells.