Region X is one of the four open reading frames (ORFs) of hepatitits B virus (HBV) and encodes a polypeptide of 154 amino acids (aa). A 584-bp BamHI-BglII fragment of the HBV DNA containing the major part of ORF X which encodes 145 aawas inserted into the BglII site within the gene II of bacteriophage M13. The insertion resulted m an in-phase gene II-X fused protein of 174 aa under the control of the gene II promoter. Cells harboring plasmids (pMLαX.59 and pMLX.12d) derived from the above construct overproduced the 19-kDa fused protein in Escherichia coli at a level of 10%-20% of total cellular protein. The fused protein was recognized by the anti-X antibodies. This is the first demonstration of using gene II promoter of M13 to express a foreign gene efficiently.