Recombinant pre-S2 peptide was purified to apparent homogeneity from E. coli by immunoaffinity chromatography using monoclonal antibody and studied for its binding characteristics with monoclonal antibody and pHSA (polymerized human serum albumin). A simple and sensitive assay method for detecting the activity of pHSA receptor on pre-S2 peptide of hepatitis B virus surface antigen (HBsAg) has been developed using pre-S2-β-galactosidase fusion protein. The assay can be used for determining pre-S2 or factors neutralizing pHSA receptor activity. The sensitivity of the assay is about 10 to 100 pmol of pre-S2.