LAS VEGAS, JULY 18 -- Unbeaten Mike McCallum unleashed a left hook that knocked out former welterweight champion Donald Curry in the fifth round tonight and retained his World Boxing Association junior middleweight title.

The end of the fight came suddenly, a little more than a minute into the round, when McCallum followed a combination by Curry with a left hook that put Curry flat on his back.

Curry managed to get onto one elbow by the count of eight, but was unable to rise any further as referee Richard Steele waved the fight to a conclusion at 1:14 of the fifth round.

"I really don't remember what happened," Curry said of the one-punch knockout. "I got careless and he hit me with the left hook."

McCallum, 30, a 2-1 underdog, scored his sixth consecutive title defense by knockout and remained undefeated in 32 professional fights with his 29th knockout.

Curry, 25, a former undisputed welterweight champion, lost for only the second time in 29 fights.

Curry was ahead on all three scorecards of the ringside judges through four rounds of the scheduled 15, and had a 40-36 shutout on one of the cards.

Curry stunned McCallum on three occasions in the early rounds, when he pressed the attack in the fifth round as his left eye began to swell shut.

McCallum, who had been trying to fight Curry from the outside, suddenly caught him off-balance after Curry threw a left hook and then a right that missed, leaving him open for the final punch.

After Steele stopped the fight, McCallum jumped on the ropes in joy as Curry's handlers rushed to tend their fighter.

"I set him up beautifully," McCallum said. "I showed him the right uppercut and moved in at the same time. I put it to the top and it was all over."

Before the round began, McCallum said, his corner "told me my pace was too fast, that I should slow it down. It was a very hard fight and I expected that. Curry is a very great champion."

The knockout moved the undefeated McCallum, boxing's longest current reigning champion, into the spotlight for the first time in his career. He immediately announced plans to move up to the middleweight ranks and challenge either Thomas Hearns or Marvelous Marvin Hagler.

Curry lost his undisputed welterweight championship last October on a sixth-round knockout by Britain's Lloyd Honeyghan.

"I just have got to go back and evaluate where to go from here," Curry said. "I'm not sure what I'll do."

McCallum admitted being hurt by Curry, including once by a left-right combination that buckled the champion's knees momentarily with about 30 seconds left in the second round.

"It was the closest to the canvas I ever came," said McCallum, who has never been knocked down.

McCallum earned his biggest paycheck ever, $500,000, while Curry earned $400,000.