The DNA sequence coding for hepatitis B virus surface antigen (HBsAg) was placed under control of the repressible acid phosphatase promoter of the yeast Saccharomyces cerevisiae in a plasmid capable of autonomous replication in both yeast and Escherichia coli. Yeast transformed by this plasmid synthesized up to 5 X 10(5) molecules per cell of immunologically active HBsAg polypeptide in phosphate-free medium. The HBsAg polypeptides produced in the yeast cells were assembled into 20- to 22-nm spherical or oval particles and were immunogenic.