JOHANNESBURG, FEB. 12 -- The prosecution of Winnie Mandela ran into serious difficulty today after two key state witnesses failed to show up in court, apparently out of fear of retribution in the wake of Sunday's mysterious disappearance of a third witness.

The latest complication immediately raised questions about whether the state would be able to proceed with the trial and whether it had been remiss in failing to protect its three star witnesses, who had been relying on the Methodist Church for their safety.

State Prosecutor Jan Swanepoel told the court he had been informed by the attorney for Kenneth Kgase and Barend Mono that the two youths were "scared to testify and may therefore refuse to testify."

Swanepoel at first said he might ask for a postponement of the trial until Monday or drop the case altogether. But at a brief afternoon session, he announced the state had decided it was "necessary to call Kenneth Kgase" and would go ahead with its case on Wednesday.

But he made it clear later that he was still not sure whether Kgase or Mono would agree to testify against Mandela, who is on trial with three codefendants on charges of kidnapping and assaulting them and two other Soweto youths. One of the four, "Stompie" Moketsi Seipei, 14, was found dead.

Immediately after the morning session in the Rand Supreme Court here, Swanepoel appeared uncertain whether he could go ahead with the trial. "I may not proceed. I don't know. Maybe the other witness will turn up, but without witnesses I can't proceed," he said.

The "other witness" is Gabriel Pelo Mekgwe, who was reported Monday to have disappeared Sunday night from the same Methodist church in the Orlando West district of Soweto where he was allegedly kidnapped with the three other youths on Dec. 29, 1988, and taken to the Mandela home for "interrogation" and a beating.

The Star of Johannesburg reported today that Mekgwe had left the church with three African National Congress men, one of them a senior official, but that it was unclear whether he had gone voluntarily.

The ANC issued a statement later denying accusations that it was "organizationally implicated" in Mekgwe's disappearance but promising to investigate them. The law should be allowed to take its course "without interference from anyone," the statement said.