Stored serum appears on the mission Remove Council lab from Operative Vargas (levels 20 - 24). There is just one of them in the mission and it will have some Council guards around. It can't move, nor has any attacks, nor any special abilities or effects. It has slightly fewer hit points than a Lieutenant of the same level. It is destructible, and any Villain or NPC caught in its blast will suffer some damage:
The contents of this crate must be destroyed!