Anti-Kryptonite was a variant of Kryptonite.
It was lethal to non-superpowered Kryptonians. Anti-Kryptonite was responsible for the deaths of the inhabitants of Argo City. It can affect even the Phantom Zoners in their ghost-like state. It is unclear if the Anti-Kryptonite in Earth-Three (which empowers Ultraman) is the same substance with same effects.