Missingno. can be traded from Red and Blue into Pokémon Yellow, but cannot be traded forward through the Time Capsule into Pokémon Gold, Silver, and Crystal. In Yellow, Missingno. levels down to level 1 if it gains any experience, and cannot grow any higher. The only way to use a Missingno. at a different level in battle is to use a link cable to battle between two games.
In the trade center, Missingno., due to its index number, appears as many different Generation II Pokémon from the perspective of Pokémon Gold, Silver, and Crystal, most commonly Tyrogue (due to the common name of ASH in Generation I). If the player tries to trade Missingno., a message will appear stating "Your appears abnormal." and the trade will automatically be cancelled. Rarely, if a Missingno. is successfully traded, it will become the Pokémon it was said to be, and act normally (though it will retain any and all moves it had as a Missingno.). At this point, it only may be traded back into the Generation I game while the player is still in the Time Capsule (where it will again become a Missingno.), as if the player leaves, the Generation II game will detect it as a Generation II Pokémon and not allow the player to enter the Time Capsule.
How fun! Source