Four viral contaminants have been routinely detected in unprocessed and commercial lots of fetal bovine serum: bacteriophage, infectious bovine rhinotracheitis, parainfluenza-3 and bovine viral diarrhea virus (BVDV). Of those, BVDV is consistently present in a majority of commercial lots of fetal bovine serum. Methods for BVDV detection and removal are reviewed. The tentative role of an unclassified pestivirus in microcephaly of infants has been reported. Its significance remains uncertain.