Contamination of animal-derived raw materials with viruses, mycoplasmas, bacteria and fungi is common. These contaminants can interfere with the diagnosis of viral infection, and vaccines produced using infected cell cultures could lead to seroconversion or disease in the vaccinated animal. The purity, safety and efficacy of viral vaccines requires testing of the ingredients, cell substrates and final product. Methods for detection of viruses, especially bovine viral diarrhea virus, in nutrient serum, cell cultures, seed viruses and viral vaccines, and the frequency of their detection at the Center for Veterinary Biologics are discussed.