Tyson stage
If you want to challenge the master, (Tyson), enter this password at the title screen under the continue option: 007 373 5963.
Beat Mike Tyson or Mr. Dream
To beat Mike Tyson, dodge his attacks for the first 1:30 of the first round and he gets weak. Now, he can still hurt you so dodge his punches (they won't kill you now) real fast and start punching him until you get him down. Continue knocking him down until you win. It may be VERY hard but keep trying !
Start At Piston Honda
At the title screen choose password selection and type in 032 730 8442. You will start fresh and clean, but fighting Piston Honda, so you skip through the first two or three other boxers.
Start At Super Macho Man
At the title screen choose password. In the password screen enter 267 913 7638. You will start fresh and clean at Super Macho Man. This let you skip through the first 5 or 6 fighters, but beware, Super Macho Man is tough to beat.
Start At The Second Title Circuit
At the title screen, choose password. Enter 005 737 5423. If you entered it correctly, you will start at the second major title circuit, skipping the first one.
Start Against Don Flamenco
At the title screen, choose password. Enter 005 373 5423. If you entered it correctly, you will start against Don Flamenco, skipping a few fighters. (Note: this looks like the second title circuit code. It's not, this one is 373, the other is 737.)
Busy signal
Enter the code 800 422 2602 and you'll hear a busy signal. This was Nintendo's old Customer Service phone number and every time you called, it was always busy.