To save labor for rationalization of serum addition conditions according to differences in sorts of cells and their lots and to reduce the quantity of expensive fetal calf serum by adding a crude drug containing saponin to a serum culture medium at a specific addition concentration.

This purpose is accomplished by adding a crude drug containing saponin to a serum culture medium at its addition concentration of ≥10μg to <100μg per ml of the medium. Cells under cultivation are given activity and their growth power is strengthened by this addition treatment and cells can be proliferated in high productivity, and therefore this treatment is useful for the production of monoclonal antibodies through hybridomas.