Leon Spinks, former world heavyweight champion, stopped Bernardo Mercado at 2:52 of the ninth round for what was billed as the American heavyweight championship.
Spinks got Mercado, who was listed as the titleholder, in trouble with a one-two combination to the head. He then knocked the 219-pound Mercado rubber-legged with two left hooks to the jaw before referee Ferd Hernandez halted the scheduled 12-round bout.
Earlier, Saoul Mamby mixed his attack beautifully to the body and the head and pounded out a one-sided decision over Termite Watkins to retain the World Boxing Council superlightweight championship.
Mamby's second defense of the 140-pound class title was on the undercard of the Larry Holmes-Muhammad Ali WBC heavyweight championship bout at Caesars Palace Sports Arena.
Judge Harold Buck of Las Vegas scored it 146-144, judge Denis Hernandez of Venezuela had it 147-141 and judge Hank Elespuru of Sacramento, Calif., saw it 147-139.
Mamby, 138 1/2 of New York, concentrated on the crouching Watkins' body in the first seven rounds. When the Houston fighter, 140, would straighten up, Mamby would stab him with jabs. Watkins did have the best of the fourth round when he landed a couple of left hooks and two good rights to the jaw, but otherwise the taller Mamby was able to block most of Watkins' hooks and tie him up inside.
It was Mamby's ninth straight win and boosted his record to 29-12-5. He won the title Feb. 23 with a 14th-round knockout of Kin Sang-hyon of South Korea in Seoul.
Watkins, who was ranked sixth by the WBC, is 46-4-1.
Meanwhile, Mark Holmes, brother of WBC heavyweight champion Larry Holmes, remained unbeaten in his young pro career by scoring a unanimous six-round decision over Randy Rivers.
Holmes ran his record to 5-0 on the undercard of his older brother's title defense against Muhammad Ali.
Holmes, 159, of Easton, Pa., showed a good left jab and in the third round landed several solid left hooks. He followed two of those hooks with hard rights to the head.
Rivers, 158 1/2, of Las Vegas, had his best round in the final three minutes, when he became aggressive and landed several awkward-appearing blows to the head.
In another preliminary, undefeated heavyweight Michael Dokes scored his ninth knockout in 20 fights by stopping Tom Fischer at 1:17 of the seventh round.
Dokes (19-0-1) won every round of the scheduled 10-rounder.
In the seventh round, Dokes landed a series of punches to Fischer's head and body. Fischer went to one knee and took the 10 count to end the fight.
The loss was the fifth in 40 fights for Fischer, who weighed 209 to Dokes' 217.