Vault City Inoculations
You have gained +10% resistance to both poison and radiation.”— Fallout 2 In-game description
This is a special perk, it can't be obtained by simply leveling up. It can only be obtained from Doctor Troy in Vault City if your Doctor skill is 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, you'll be able to ask for this perk.
This perk appears only in Fallout 2.