The following are known Action Replay Codes for Mega Man & Bass on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
77108ebd
772fdb65
e4ec44bb
a66df8e5

Infinite Health
6e052154
9c826158

Infinite Lives
8acbe27f
e0ff162e

Max Screws
12378cc4
fc89cb2a

Press L & R: Hover
cbb5799d
0ca4d71b
83d35f70
613f88c9

Have All Items
c382a86c
82ee4d29
2aed3d64
7205d4ff

Have All CDs
bba486eb
ab059b70
e482cd99
588e41ff
3ad30657
7b7dd3d7

Have All Weapons
ef5b8e43
45d0f841
9d2d8c98
3f7bb4d2