Jamieson, Fausset and Brown on Mark 8:15

15. And he charged them, saying, Take heed, beware of the leaven of the Pharisees--"and of the Sadducees" (Mt 16:6).

and of the leaven of Herod--The teaching or "doctrine" (Mt 16:12) of the Pharisees and of the Sadducees was quite different, but both were equally pernicious; and the Herodians, though rather a political party, were equally envenomed against our Lord's spiritual teaching. See on Mt 12:14. The penetrating and diffusive quality of leaven, for good or bad, is the ground of the comparison.