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When did kryptonite first appear in Superman stories?

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Christopher Reeve as Superman in a blue suit with red cape and yellow belt, flying with one arm forward against a dark background in a publicity still from Superman: The Movie. — Superman Christopher Reeve in *Superman* (1978), directed by Richard Donner.

Kryptonite first appeared in the Adventures of Superman radio show in 1943, where it was introduced as a radioactive mineral from Superman's home planet that could weaken him. While the term appeared in newspapers shortly after its radio debut, it did not make its first appearance in the written form of the Superman comic books until 1949.12