Abstract: In accordance with the present invention, recombinant plasmids are described which, when inserted into microbial hosts, direct the synthesis of proteins in which preS polypeptides found on the surface of human hepatitis B virus are fused to the enzyme .beta.-galactosidase. The recombinant plasmids are produced by inserting DNA sequences encoding the preS polypeptides into the lacZ gene, which codes for E. coli .beta.-galactosidase, carried by the plasmids. Large amounts of the preS-.beta.-galactosidase fusion proteins can be isolated from microbial cultures carrying the recombinant plasmids. Antigenic determinants of fusion protein so produced are recognized by antibodies to the preS determinants of native hepatitis B virus. The .beta.-galactosidase activity of such fusion protein is detected by a suitable chromogenic or fluorogenic substrate. PreS-.beta.