From: Mark 8:14-21
The Leaven of the Pharisees (Continuation)
 Now they had forgotten to bring bread; and they had only one loaf with them in the boat.  And He (Jesus) cautioned them, saying, "Take heed, beware of the leaven of the Pharisees and the leaven of Herod."  And they discussed it with one another, saying, "We have no bread."  And being aware of it, Jesus said to them, "Why do you discuss the fact that you have no bread? Do you not yet perceive or understand? Are your hearts hardened?  Having eyes do you not see, and having ears do you not hear? And do you not remember?  When I broke the five loaves for the five thousand, how many baskets full of broken pieces did you take up?" They said to Him, "Twelve."  And the seven for the four thousand, how many baskets full of broken pieces did you take up?" And they said to Him, "Seven."  And He said to them, "Do you not yet understand?"
15-16. In another Gospel passage--Luke 13:20-21 and Matthew 31:33--Jesus uses the simile of the leaven to show the vitality of His teaching. Here "leaven" is used in the sense of bad disposition. In the making of bread, leaven is what causes the dough to rise; the Pharisees' hypocrisy and Herod's dissolute life, stemming from their personal ambition, were the "leaven" which was poisoning from within the "dough" of Israel and which would eventually corrupt it. Jesus seeks to warn His disciples about these dangers, and to have them understand that if they are to take in His doctrine they need a pure and simple heart.
But the disciples fail to understand: "They weren't educated; they weren't very bright, if we judge from their reaction to supernatural things. Finding even the most elementary examples and comparisons beyond their reach, they would turn to the Master and ask: `Explain the parable to us.' When Jesus uses the image of the `leaven' of the Pharisees, they think that He's reproaching them for not having purchased bread....These were the disciples called by our Lord. Such stuff is what Christ chose. And they remain just like that until they are filled with the Holy Spirit and thus become pillars of the Church. They are ordinary people, full of defects and shortcomings, more eager to say than to do. Nevertheless, Jesus calls them to be fishers of men, co-redeemers, dispensers of the grace of God" ([St] J. Escriva, "Christ Is Passing By", 2). The same thing can happen to us. Although we may not be very gifted, the Lord calls us, and love of God and docility to His words will cause to grow in our souls unsuspected fruit of holiness and supernatural effectiveness.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.