In a previous study, we identified that transferrin receptor (TfR) is the receptor utilized by hepatitis B virus (HBV) to enter T cells. We demonstrated that hepatitis B envelope antigen (H-BenvAg) is taken up by activated T cells via TfR, processed in endosomal compartments, and presented on class II molecules to specific CD4(+) T cell clones. Herein, we report that binding to soluble ferric Tf by HBenvAg is needed in TfR-mediated endocytosis. Accordingly, presentation of HBenvAg by activated T cells is not observed in serum-free medium and is restored by addition of soluble Tf. Moreover,we provide evidence that pre-S2 and S regions of HBenvAg contain the critical residues for the interaction with soluble Tf. Our data not only explain HBV entry into a variety of host activated cells, but may also help in developing strategies to alter the course of chronic HBV infection.