ATLANTIC CITY, N.J. — Mike Tyson kept his mind on his business Monday night. And business was booming.

Tyson (35-0, 31 KOs) bombarded Michael Spinks with a barrage of left hooks, then polished him off with a hard right at 1:31 of the first round to retain his WBC, WBA and IBF heavyweight titles.

It was one of the shortest heavyweight championship fights in history. Only three title bouts have ended quicker.

“I had him hurt before,” said Tyson, who is the first man ever to floor Spinks. “From what I know of human nature, he was either going to run and hide, or get back and fight. He tried to come back and fight, and I was ready for him.”

By beating Spinks (31-1, 21 KOs), Tyson vanquished the one man who had a right to dispute his claim as the greatest fighter on Earth.

“I wanted to box him, but I thought I had to nail him a couple of times to get his respect,” said Spinks, the only light heavyweight to win the heavyweight championship. “I just came up short.”

Monday, just three days before his 22nd birthday, Tyson made a wish and cut into Spinks, who will be 32 next month.

“As far as I know, this might be my last fight,” Tyson said.

His all-out assault was simply too much for Spinks, who was stripped of his IBF championship last year when he fought Gerry Cooney rather than enter HBO’s title unification series.

On the other hand, the sellout crowd of 21,788 who contributed to the 12.3 million live gate to make this the highest grossing sports event in history, couldn’t get enough.

For his pain, Spinks got a guarantee of $13.5 million. Tyson’s cut from the fight, which was televised on closed-circuit and pay-per-view television, would be in the neighborhood of $20 million.

Using his vaunted left, Tyson put instant pressure on Spinks, finally connecting with a left uppercut and following with a right to the body to put Spinks on the canvas for the first time in his career.

“I saw it in his face when I came in the ring,” Tyson said. “I knew it would be a first-round knockout.

“The first punch I threw wobbled him. There were two things he could do, either try to get lucky with a cut or run all night.”

When Spinks regained his feet, Tyson stepped up the pressure. Spinks missed with an overhand right, and Tyson missed with his counter left.

But as the left flicked out, Tyson followed with a short right to the head. That dropped Spinks for good at 1:31 of the first round.

Referee Frank Capuccino began to count and it was obvious the stricken Spinks wouldn’t beat it.

“Whatever he does, I’m going to win,” Tyson had said before the scheduled 12-round bout. Spinks never got a chance to do anything.

“No one on the planet can come near me,” Tyson had boasted. Certainly no heavyweight who is currently campaigning.

Tyson was a 4-1 favorite. It turned out the odds were way out of line.

The only faster knockouts in heavyweight title history were Jim Jeffries’ 55-second victory over Jack Finnegin in 1900, Michael Dokes’ 1:03 win over Mike Weaver in 1982 and Tommy Burns’ 1:28 defeat of Jem Roche in 1908.

There was speculation of squabbling in Tyson’s camp and reports that marital problems might trouble him. He answered that question swiftly and ruthlessly.

Spinks had said before the fight, “I don”t know what kind of problems Mike Tyson will present… As soon as the fight ends, I’ll be able to tell you.”

Spinks was in no condition to tell anyone anything at the end of this fight.

When Spinks did come around, he said, “I finally found someone who can beat me.”

“As a light heavyweight I went against guys faster than Mike,” Spinks added. “I’ve been against speed. I wasn’t intimidated. I had to do the necessary thing to avoid getting hit.

“When you take an eight count to indicate you’re hurt, he comes in and tries to nail me with a good right hand. I tried to exchange punches with him but he countered me with an uppercut.”

Spinks was trying to become the third former champion to regain the heavyweight title. He became the 15th to fail.

In boxing’s financial district and in the ring, Tyson is the king of the neighborhood.

At 20, he won a piece of the title and just one month past his 21st birthday became the first undisputed champion since Spinks’ older brother, Leon, beat Muhammad Ali in 1978.

Where Mike Tyson is concerned, you can underscore undisputed.

TYSON’S RECORD

Mike Tyson’s record is 35-0 with 31 KOs.

1985

March 6 Hector Mercedes Albany, N.Y. KO 1

April 10 Trent Singleton Albany, N.Y. KO 1

May 23 Donald Halpin Atlantic City, N.J. KO 4

June 20 Rick Spain Atlantic City, N.J. KO 1

July 11 John Anderson Atlantic City, N.J. KO 1

July 19 Larry Sims Poughkeepsie, N.Y. KO 3

Aug. 15 Lorenzo Canady Atlantic City, N.J. KO 1

Sept. 5 Michael Johnson Atlantic City, N.J. KO 1

Oct. 9 Donnie Long Atlantic City, N.J. KO 1

Oct. 25 Robert Colay Atlantic City, N.J. KO 1

Nov. 1 Sterling Benjamin Latham, N.Y. KO 1

Nov. 13 Eddie Richardson Houston KO 1

Nov. 22 Conroy Nelson Albany, N.Y. KO 2

Dec. 6 Sam Scaff New York KO 1

Dec. 27 Mark Young Colonie, N.Y. KO 1

1986

Jan. 11 David Jaco Albany, N.Y. KO 1

Jan. 24 Mike Jameson Atlantic City, N.J. KO 5

Feb. 16 Jesse Ferguson Troy, N.Y. KO 6

March 10 Steve Zouski Uniondale, N.Y. KO 3

May 3 James Tillis Glen Falls N.Y. W 10

May 20 Mitchell Green New York W 10

June 13 Reggie Gross New York KO 1

June 28 William Mosea Troy, N.Y. KO 1

July 11 Lorenzo Boyd Swan Lake, N.Y. KO 2

July 26 Marvis Frazier Glen Falls, N.Y. KO 1

Aug. 17 Jose Ribalta Atlantic City, N.J. KO 10

Sept. 6 Alfonso Ratliff Las Vegas, Nev. KO 2

Nov. 22 Trevor Berbick Las Vegas, Nev. KO 2 (a)

1987

March 3 James Smith Las Vegas, Nev. W 12 (b)

May 30 Pinklon Thomas Las Vegas, Nev. KO 6 (COLOR)

Aug. 1 Tony Tucker Las Vegas, Nev. W 12 (d)

Oct. 16 Tyrell Biggs Atlantic City, N.J. KO 7 (e)

1988

Jan. 22 Larry Holmes Atlantic City, N.J. KO 4 (e)

March 20 Tony Tubbs Tokyo KO 2 (e)

June 27 Michael Spinks Atlantic City, N.J. KO 1 (f)

(a), won the WBC heavyweight title, (b), won the WBA heavyweight title, (COLOR), retained the WBA-WBC heavyweight title, (d), retained the WBA-WBC heavyweight title and won the IBF title, (e), retained the World heavyweight title. (f), retained the World heavyweight title.