Mark 8:15 - Then He charged them, saying, "Take heed, beware of the leaven of the Pharisees and the leaven of Herod." [NKJV]
In the Bible, "leaven" is used both literally (the yeast that makes bread rise) and figuratively (a relatively small force or amount that nonetheless makes a major impact). As used by Jesus in Mark 8:15, "leaven" refers to a "corrupting influence."