“of the Pharisees.” The Gospel of Matthew adds the Sadducees as well (Matt. 16:6); they had different doctrines but both were erroneous.
“the leaven of Herod.” There are various opinions about the meaning of the leaven of Herod, and there may be an element of truth in all of them, because Jesus was warning the apostles about things that would make their ministry ineffective or even against God. Both the Pharisees and Herod wanted “proof” (a sign) that Jesus was who he said he was, and both refused to believe the evidence that was right before their eyes. Also, the Pharisees, Sadducees, and Herod all doubted who Jesus was and spoke lies about him (Herod, for example, said that Jesus was John the Baptist raised from the dead). So each party failed to understand the ministry of Jesus or take the man and his ministry at face value as being from God. It is also interesting that Matthew says “Pharisees and Sadducees,” while Mark says “Pharisees and Herod.” One of Herod’s wives was Mariamne, the daughter of Simon who once was the High Priest and was a Sadducee, so the family of Herod was connected with the Sadducees by marriage.