The gene coding for hepatitis B large envelope protein was cloned under the lac promoter in bacterial vector pUC-8 and under the ADH1 promoter in yeast expression shuttle vector pVT103-U, and expression of HBsAg in bacteria and yeast was determined. The strongest expression of large envelope protein was obtained after transformation of the protease-deficient yeast strain BJ1991. The recombinant large envelope protein did not form complex 22-nm particles and was not secreted into medium.