The antiproton injection subsystem, or antimatter injection subsystem, was one of the engineering systems of Federation starships. It was an integral component of the warp propulsion system. This subsystem connected the antimatter chamber to the dilithium regulation chamber. In this subsystem, the antiproton injectors were located above the chamber, and the antiproton, or antimatter, was streamed through the antimatter stream coils. Safety measures built into this subsystem included antiproton injection seals. During an emergency situation, if there were antiprotons in the subsystem, antiproton injection seals on the dilithium regulation chamber were designed to prevent these particles from entering into this chamber and mixing with the deuterium. Aboard Galaxy-class starships, this subsystem was located on Decks 37 to 41. The location of this subsystem was labeled in the schematic of the secondary hull of the USS Yamato in 2365.
Aboard the Galaxy-class USS Enterprise-D, in the Engineering Systems Database, there was a folder titled Antimatter Injection System Operaton. This folder contained technical manuals relevant to the topic. (TNG: "Contagion", "Booby Trap", okudagrams)