Replacement for lumbar reservoir. Water can often be the heaviest thing in your pack. The CamelBak® Lumbar Reservoir platform allows you to carry that weight on your hips, as opposed to your shoulders. *UPGRADE AVAILABLE: Please note that the Antidote® Reservoir has been replaced by the new CRUX™ Reservoir – offering new features and enhancements including 20% more water with every sip. We continue selling Antidote on CamelBak.com to those customers looking for a reservoir replacement that will fit perfectly in the reservoir compartment of their older Antidote-designed backpacks.