The production kinetics of hepatitis B surface antigen (HBsAg) at different stages of the fermentation of a recombinant Saccharomyces cerevisiae strain are described. Antigen accumulation inside cells was dependent on cell growth rate and a low cell growth rate resulted in a high HBsAg accumulation rate. HBsAg production per cell at the end of fermentation increased by 32% on supplementing with vitamins and limiting cell growth. (C) 2003 Elsevier Science Ltd. All rights reserved.