Immune: undead, constructs, elementals, and any creature with fast healing or regeneration

A creature that is taking bleed damage takes the listed amount of damage at the beginning of its turn. Bleeding can be stopped by a DC 15 Heal check or through the application of any spell that cures hit point damage (even if the bleed is ability damage). Some bleed effects cause ability damage or even ability drain. Bleed effects do not stack with each other unless they deal different kinds of damage. When two or more bleed effects deal the same kind of damage, take the worse effect. In this case, ability drain is worse than ability damage.

Marius, Scholai Armsmaster says: Be mindful that undead, constructs, and elementals are all immune to bleed damage, as is any monster with fast healing or regeneration. Bleed damage is particularly effective on animals, as they lack both the wits and the thumbs to perform Heal checks on themselves.