You have gained +10% resistance to both poison and radiation.”— Fallout 2 In-game description
This is a special perk, it cannot be obtained by simply leveling up. It can only be obtained from Dr. Troy in Vault City with a Doctor skill of at least 75%, and only after having received the Vault City Training perk. After receiving the the Vault City Training perk, upon going to Vault 8 to treat poisoning or radiation, this perk can be asked for.
This perk appears only in Fallout 2.