We have expressed both S and preS2-S genes coding for the hepatitis B small (S) and medium (M) proteins, respectively, in different yeast based expression systems and compared the production level of the recombinant proteins. In Saccharomyces cerevisiae, viral genes were expressed under the inducible Gal10/cyc1 and the constitutive PGK promoters using 2 mu replicating vectors. We showed that the yield of S protein was higher than M protein under both inducible (14.27 vs 10.9 mg/l) and constitutive (9.18 vs 6.39 mg/l) conditions, respectively. In the methylotrophic yeast Pichia pastoris, the viral genes were expressed in GS115 (Mut(+): Methanol Utilizing) and KM71 (Mut(S): Methanol Utilizing Slow) under the control of the alcohol oxidase promoter (AOX1). In Mut(S) background, both S and preS2-S genes were expressed at higher levels than in Mut(+). In attempt to increase the yield of recombinant viral proteins in S. cerevisiae, we have co-expressed both inducible and constitutive vectors harboring the S or preS2-S genes leading to recombinant strains called UTS (containing pDP/S+ pYePIT/S) and UTP (containing pDP/preS2-S + pYePIT/preS2-S). We showed that the recombinant S and preS2-S proteins were successfully detected and the production level reached 18.31 mg/l for the S and 13.22 mg/l for the M proteins. Our comparative study provides evidence that in small scale, S. cerevisiae is more suitable for HBsAg and preS2-S proteins production than P. pastoris under inducible rather than constitutive condition. (C) 2009 Elsevier Inc. All rights reserved.