BACKGROUND/AIMS: The introduction of Hansenula polymorpha for recombinant hepatitis B vaccine production allowed high product yield with plasmid stability and less glycosylation than conventional Saccharomyces cerevisiae system. A Green Cross HG-II vaccine formulated from HBsAg produced by a recombinant strain of the yeast H. polymorpha was evaluated for immunogenicity and safety in an open label triaL METHOFD: A 20 ug dose of Green Cross HG-II vaccine was administered intramuscularly at 0, 1 and 6 months at the deltoid region in 118 healthy adults seronegative for HBV markers. The anti-HBs titers were determined at one month after administration of the third dose of vaccine by radioimmunoassay. RESULTS: The seroconversion rate was 96.8% (90 out of 93), with seroprotective rate of 95.7% (89 out of 93). The geometric mean titers(GMT) of the anti-HBs response was 153.1mIU/ml in seroconverters. An age-dependent effect was observed in the anti-HBs response. But sex-dependent effect was not prominent. Reactogenecity was in incidence and general reactions were short-lasting and a mainly mild in severity. CONCLUSIONS: The results of this study have shown that the Green Cross HG-II vaccine is safe and clinically well tolerated, a nd that it may provide protection against HBV infection.