Enter the following codes at the "Press Start" screen:
Cheat Mode:
Square, Circle, L2, R2 to unlock all modes, boxers and arenas.
Big Head Mode:
Square, Circle, Up, Down.
Unlock Flat Mode:
Down, Up, Circle, Square.
Unlock New Boxer Textures:
L1, R1, X, X, Triangle, X.
Unlock Small Head Mode:
Square, Circle, Down, Up.
Unlock Super Mutant Mode:
Square, Left, Up, Triangle.
Unlock Mike Tyson Challenge:
Beat Tyson and earn the gold belt.
Unlock All Fighters:
Get Bronze, Silver and Gold belts to unlock all fighters.