John Tate, the undefeated American heavyweight, stamped himself a rising threat in the division when he stopped South African Kallie Knoetze at 2:30 of the eighth round in their scheduled 12-rounder today.

Tate, the 24-year-old slugger from Knoxville, Tenn., raised his record to 19-0 as he punished Knoetze with thudding lefts and rights to the face, shattering the 26-year-old South African's hopes for a shot at the world heavyweight title.

The mutton-chopped former policeman who weighed 225 pounds, was bleeding profusely from under his right eye and nose when referee Isidoro Rodriguez of Venezuela stopped the onesided fight.

Knoetze, the No. 1-ranked WBA contender, began the bout by charging at the heavier American like one of the bulls that were roaming the bush outside the open-air soccer stadium.

But most of his punches went wild. Tate, the No. 3-ranked contender who weighed 233 pounds, continually danced away from the lumbering South African.

By the Seventh round, Tate, a black, was openly winking and smiling at the predominately black crowd of 50,000 spectators in this country that was granted independence by apartheid South Africa. The United States, which does not recognize Bophuthatswana, claims South Africa's white minority merely formed a country to take heat off the apartheid issue.

Neither Tate nor Knoetz would talk with reporters immediately after the fight.

Knoetze had been a target for demonstrators when he fought in Miami earlier this year. Working as a policeman, Knoetze had once shot a black child, who allegedly was fleeing from a robbery, in the leg.

In preliminary fights, Harold Volbrecht of South Africa defeated Johnny Gant of Washington, D.C., in a welterweight bout; South African middleweight champion Elijah Makhatini knocked out Gary Guiden of Muncie, Ind., in the fourth; heavyweight Jimmy Abbott of South Africa knocked out Dave Johnson of Bowie, Md., in the third round, and Mike Schutte of South Africa fought to a draw with Dwain Bonds of Detroit in an eight-round heavyweight bout. CAPTION: Picture, South Africa's Kallie Knoetze covers up from right hand by John Tate, who stopped his opponent in eighth round of their heavyweight bout in Bophuthatswana. AP