Sega Master System Cheats - Alien 3

Unlimited lives:

Start game play in two player mode and obtain a high score for both players. Intentionally have one of the players lose all their lives and enter ALIEN as a name on the high score screen. The corresponding player will have unlimited lives when game play continues.

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Level select:

Press Up, Down, 2, Left, Right, 2, Up, Down, 2 when the "Start" and "Configure" options appear at the title screen (before the Alien 3 logo disappears).

Unlimited lives:

Start game play in two player mode and obtain a high score for both players. Intentionally have one of the players lose all their lives and enter "ALIEN" as a name on the high score screen. The corresponding player will have unlimited lives when game play continues.

Extra ammunition:

Start a game in two player mode and intentionally die, then switch to single player mode.