Adenosine 5'-monophosphate was found effective in enhancing hepatitis B surface antigen (HBsAg) production in the culture of recombinant Chinese hamster ovary (CHO) cells, but it markedly inhibited cell growth. A culture strategy with periodical medium changes was established to increase HBsAg production and avoid the AMP-induced inhibition of cell growth. The enhanced energy supply might be one of the reasons for the increase in HBsAg production in the presence of AMP.