The effect of serum on recombinant protein production using vaccinia virus expression system was studied in both monolayer and suspension cultures of HeLa S3 cells. A model heterologous protein was $\beta$-galactosidase. Calf and horse sera were used, and their concentrations were 0.5-10\%(v/v). In monolayer culture, calf serum did not show any significant influence on $\beta$-galactosidase production. Horse serum, on the other hand, resulted in higher amount of product at 10\% concentration and its level increased continuously up to 96 hr p.i. Maximum total $\beta$-galactosidase values were $3.6 \pm 0.7 I.U./mL$ for calf serum and $2.8 \pm 0.7 I.U./mL$ for horse serum. In suspension culture, both serum showed similar pattern of their own as in monolayer culture. Maximum enzyme levels were 3.3 I.U./mL for calf serum and 3.6 I.U./mL for horse serum. Both sera showed lower maximum value, compared with monolayer culture. Cell numbers were also lower in suspension culture. However, enzyme activity per million cells were about 1.5-fold higher than that in the monolayer culture.