Mike Tyson, the 19-year-old street kid from Brooklyn who never has lost a professional heavyweight fight, won another one tonight with a unanimous, if unspectacular, 10-round decision over 29-year-old Mitch Green at Madison Square Garden.

Pounding away incessantly inside the huge bear hug of the 6-foot-5 Green, Tyson won his 21st consecutive professional fight in a plodding, workmanlike fashion that he clearly enjoyed more than the 6,259 spectators did.

"I was having a ball out there," Tyson said. "It was my best fight ever. I love to go 10 rounds."

Other than the three times that Tyson uppercuts knocked Green's mouthpiece out of the ring, the most unusual thing about this fight was how long it took to complete.

Up until this month, Tyson had won each of his pro fights by knockout, 13 in the first round. But on May 3, he went 10 rounds before winning a decision against James Tillis, and now this.

"Not to be egotistical, but I won the fight so easily it was easy," said Tyson, undaunted.

It wasn't particularly difficult -- he was right about that. It just took some time. Judges George Colon and Pat Dolan scored the fight 9-1. Judge George De Gabriel had it 8-2.

Green (16-2-1) saw it another way: "No desire, no determination, no nothing," he said angrily. "I couldn't get motivated because I'm not getting paid."

A scene perhaps more interesting that the fight itself occurred Monday at the weigh-in. Green screamed at his manager, Carl King, promoter Don King's son, after he discovered Tyson was guaranteed almost seven times the amount Green will be paid for fighting. Tyson will get $200,000, Green about $35,000.

"As soon as I found out the payday, I couldn't fight," Green said tonight.

Tyson arrived in the ring tonight as he always does, wearing only the essentials: trunks, shoes and gloves. He never wears a robe. He stood in the middle of the ring as Green danced in his white robe, his long, straightened hair bouncing with each step. Green was hardly sweating.

That soon changed. Tyson, who is 5-11 and, at 215 pounds, 10 pounds lighter than Green, dashed out at the sound of the bell, immediately on the attack. He wasn't flashy. It was the beginning of the bobbing, rib-smashing strategy for which Tyson has become known. Statisticians recorded that Tyson landed 69 percent of his body punches this evening, which is considered quite good.

Early in the second round, Tyson sent Green's mouthpiece flying with a quick right uppercut. Most of the action was inside, as Tyson continued to drill Green's body, landing bunches of side punches.

Green, apparently unable to tie up Tyson inside, was pummeled by Tyson's vaunted inside punching from the middle of the first round on. Tyson burrowed in at every opportunity, wedging his head under Green's chin and batting away at Green's body with his free hand.

After the first round and again after the fourth and sixth, Tyson and Green wouldn't stop at the bell. They continued to go after each other, leering and punching until referee Louis Rivera stepped between them.

Tyson began each round exuberantly, running out to meet Green, who was beginning to look particularly weary by the third round. Green just stood and watched him run.

Halfway through that round, Tyson jackhammered a tremendous left uppercut to Green's mouth, sending his head popping back -- and his mouthpiece flying once again. One of Green's incisors from his dentures also was sent out of the ring, landing in a reporter's lap.

Tyson jarred the mouthpiece out of Green's mouth one final time in the 10th and last round.

Green came alive briefly in the fifth round with a flurry of back-pedaling punches, but the rally appeared to tire him out as much as fire him up. By that time, the fight was just about over.

Yet the best exchange of the fight occurred in the eighth round. Green landed a combination of punches, most of which found Tyson's shoulders and gloves. Tyson then responded with a couple of industrial-strength uppercuts, one of which picked up Green right off the canvas.

"Everybody is going around saying he's just a boy," Tyson said, "but I'm going to be the only boy to be king of the world."