Begin a regular game then press A + B + Y + X on Controller Two. Next, press one of the following buttons on Controller One to activate the cheats:
Press A for Invincibility
Press B to do more damage
Press X for Unlimited Weapons
To turn off any of the cheats, simply press the button that
activated the option one more time.
You can tell which cheats are active by looking at the numbers in the left-hand corner of the screen: '1' indicates invincibility, '2' indicates damage, and '4' indicates unlimited weapons. If combinations of cheats are active at once, the number totals. For example, if all are active, you'll see the number "7."
Stick Ripley into the middle of any level with these spacey passwords: