If either fetal or adult bovine serum is incorporated in the nutrient medium of cell cultures used to propagate PPV, it should be pretested for viral inhibitors. The same may apply to sera of several other species. Because replication of PPV is affected by mitotic activity, the effect of the serum on the cells is also especially important. In addition, cultures should be pretested for PPV contamination. Cultures are sometimes unknowingly prepared from infected tissues of fetal and postnatal pigs. Moreover, PPV can be accidentally introduced into cultures in several ways, including the use of contaminated trypsin. If contamination is detected before all cells are infected, the virus can be eliminated by repeatedly subculturing the cells in the presence of nutrient medium containing PPV antiserum.