Then Jesus said unto them,.... Either taking occasion from the disciples observing that they had forgot to take bread with them, or on account of what passed between him and the Pharisees and Sadducees, he gave the following advice to his disciples;take heed and beware of the leaven of the Pharisees and of the Sadducees.
Mark, instead "of the leaven of the Sadducees", says, "the leaven of Herod"; either because Christ might caution against all three; or because the Sadducees were generally Herodians, taking Herod to be the Messiah; or were on his party, or for his government, which the Pharisees disliked; and the Herodians were generally Sadducees.
By "the leaven" of these is meant their doctrine, as appears from Mt 16:12.