The surface antigen gene of HBV (adr subtype) is inserted into the YEP 1 which is replicated and selected in E. coli and baker yeast Saccharomyces cerevisiae. The HBsAg gene can be expressed in yeast under GAL-10 promoter control. The protein synthesized in yeast is assembled into particles that have the same size and shape as particles isolated from plasma. One microgram surface antigen can be yielded in 100 ml culture.