PURPOSE: To prevent the bacteriolysis of host caused by viruses, and to cultivate a cell or microorganism smoothly without inhibition, by adding serum containing an antibody to neutralize the viruses to a medium, cultivating the cell or microorganism.

CONSTITUTION: In cultivating a cell or microorganism, serum containing an antibody to neutralize viruses is added to a medium. A cell derived from mammals or various kinds of hybridomas prepared by cell fusion may be cited as the cell. Ray fungs, bacteria, etc. may be cited as the microorganism. A rabbit is suitable as the animal because of easy acquisition and a large amount of blood, and a guinea pig, rat, dog, sheep, etc. may be used. As to period to add the serum containing an antibody to neutralyze the viruses to the cell or microorganism, any period during cultivation may be available, and the addition at the beginning of cultivation is especially preferable.