Michael Spinks still is the International Boxing Federation heavyweight champion of the world tonight, settling for the moment all doubts and quieting his critics. He did it tonight with a split decision over a true but aging heavyweight, Larry Holmes.

Spinks, who last September decisioned Holmes to become the first light heavyweight to dethrone a heavyweight champion, won a narrow decision from two of three judges at the Hilton Center. In a fight that easily could have gone the other way, and very nearly did, he defeated Holmes with a furious late resurgence after the slow and tired challenger had claimed the early rounds with an attack that defied his 36 years.

Judge Frank Brunette scored the fight 144-141 for Spinks, and judge Jerry Roth scored it 144-142. The dissenter was judge Joe Cortez, who scored it 144-141 for Holmes. (The Associated Press had it 143-142, Holmes.)

Spinks, who earned $2.5 million with his victory, weighed in at 205 pounds, five more than in September and 30 more than as a light heavyweight. His elusive, awkward style preserved him only partially from the 223-pound Holmes' early punches, the challenger clearly taking the first five rounds. But Spinks turned the fight in his favor in the middle rounds, then won two of the last three rounds decisively, using a relentless stream of jabs and flurries.

"I left him behind," Spinks said. "I had to. Larry showed determination, that he wasn't over the hill. He showed he wasn't going to give me anything. I had to break his morale. I showed him that, once he had his fun, I was going to catch him and pass him."

It was the second time in seven months Holmes lost a decison to Spinks after his seven-year, uninterrupted reign as champion. Spinks' victory in September also cost Holmes his chance to tie Rocky Marciano's record of 49-0, and Holmes was an embittered loser again tonight.

"I did it and they said I couldn't," Holmes said. "I can go home and be proud."

Holmes was soaking his right hand in ice after the fight. He did not appear for the interviews, making his remarks only to HBO. Nearly 1 1/2 hours after the fight, he was taken to a local hospital to find out if he had broken his right thumb. A spokesman said Holmes hurt the thumb in the third round.

Holmes, who won $1.5 million, implied that it was his last fight. He also implied that the criticism he has received for his occasionally unpopular statements was one of the reasons why.

"There's no such thing as fairness in the world," he said. "I gave it all I had. I hope I left a lasting impression on boxing. There's no sense chasing ghosts. I'd be punched out, punch drunk, stuttering.

"I can't win no more. Because I represent the rightness of boxing, I call things the way I see them. When I speak out, I get penalized."

There is no doubt he has declined physically, although he fought substantially better than in September, when he appeared barely able to raise his all-important right hand.

At 29, Spinks used youth and stamina simply to wear Holmes down.

For all of Holmes' criticism of the judging, however, he may well have gotten some undeserved benefit of the doubt from one judge. Cortez, the judge who voted for Holmes, called the final round even. Most observers called it perhaps Spinks' best round of the fight, and most decisive.

Spinks moved in aggressively in the 15th and caught Holmes with two quick jabs to the head, beating him to the punch. When Holmes retreated to the ropes, Spinks followed and lashed out repeatedly, trying for the knockout that he knew would keep him from worrying about the scoring. But Holmes at that point appeared to be merely a survivor.

"I knew he'd wear down," Spinks said. "I was beating him half to death inside. I made him eat a lot of jabs."

As the fight moved through the early rounds, it appeared that Spinks was surprised by Holmes' aggression, and that Spinks was resting too long on his defensive skills. He would need to hurry with new tactics lest he fall too far behind.

Holmes, as he had promised, came out smoking. He rushed Spinks, landing hard to the body and then wrestling the champion to the canvas in midround. It was typical of Holmes' attack in each of the first five rounds, in which he piled up points steadily by crowding Spinks, who fought mainly on the defensive, either from desperation or for strategic purposes.

In the sixth, however, Spinks showed a sudden change of tactics. He won a round for the first time when he took the offensive with three solid left hooks to Holmes' head, getting no reply. At that point Holmes showed his first signs of slowing, and it portended the swing of momentum.

The turning point came in the eighth round. Spinks came out jabbing and then delivered a hard right cross that shook Holmes, who retreated. Spinks followed up with more shots to the head for his most convincing round of the fight. He finished it with seven straight jabs to the face.

From the ninth through the 12th it was a new Spinks, who willingly became the aggressor. In the 10th round, he landed with both hands, scoring again to the head. Another two-handed barrage left Holmes obviously in trouble, and he welcomed the bell.

Thanks to his early lead, Holmes still was ahead on points, but Spinks scored again heavily in the 13th with Holmes' fatigue showing and the challenger now on the defensive.

But then Holmes threatened to pull the fight out with one punch in the 14th. He caught Spinks unaware with a hard, short right to the face that staggered the champion in mid-round. Spinks' knees buckled and he appeared on the verge of going down. Here Spinks showed his flair for survival. He recovered to flurry at Holmes' head, although it remained a losing round.

"He stunned me at least twice," Spinks said. "All I knew was I was standing in one place for too long. I said, 'Let me get out of here,' and I dashed on him. I knew things were hazy, but I wasn't going anywhere without my luggage."

The final round was all Spinks. The champion beat up on the now listless Holmes, scoring on his head with both hands from midround until the closing bell with scarcely a reply. Spinks' late round rallies left it in the laps of the judges.

Spinks claimed after the final bell and before the judges scoring was in, that Holmes said to him: "You took the fight."