Biomass is biological matter that can be used by the zerg in order to evolve the Swarm.
Left 2 DieEdit
In the official arcade map Left 2 Die, biomass is used as a resource, and is obtained by killing special zerg units and destroying infested buildings. A science facility can be used to spend biomass in new units, structures and upgrades.
Heart of the SwarmEdit
In the Heart of the Swarm campaign, biomass is used both for primary mission objectives and as optional objectives to gain levels for Kerrigan, effectively replacing mutagen from earlier in development.
In Co-op Missions, Abathur gains biomass from destroyed enemy units. This allows the creation of upper tier units such as the brutalisk and leviathan. Abathur's combat units (excluding overseer) gain increased 3% life, 1% attack speed, and 5% energy regeneration per stack of biomass. The drop rate of biomass depends on the mission difficulty, on casual and normal, it will drop 5 biomass per supply, on hard it will be 4, and on brutal it will be 2.5. Biomass in a range of 4 of each other will merge into one pickup. Neutral critters also generate biomass (however, killing a critter by "critter nuke" won't generate one).
On an island on the east side of the map "Old Soldiers", there is an outhouse labeled "Terran Biomass."