WorldCom's former chairman has refused to answer questions from a congressional panel investigating accounting irregularities at the company.
Mr Bernard J Ebbers invoked the US constitution's Fifth Amendment.
WorldCom's former chief financial officer, Mr Scott Sullivan, also refused to testify, "based upon the advice of counsel."
Mr John Sidgmore, WorldCom's president and chief executive officer, blamed the company's former management for the accounting problems.
"I do not believe I have anything to hide in these or any other proceedings," Mr Ebbers told the House Financial Services Committee.
He said he'd been advised by his Washington attorney, Mr Reid Weingarten, to remain silent because of the range of investigations by the Justice Department and Securities and Exchange Commission
"WorldCom uncovered this problem internally," Mr Sidgmore said in prepared testimony. "The kind of initiative demonstrated by our internal audit group is to be applauded and will continue to be encouraged."
WorldCom Chairman Mr Bert Roberts called the accounting improprieties "an outrage to me," and said auditor Mr Arthur Andersen was responsible. "To my mind, the failure of our outside auditors to uncover them is inconceivable," he said.
Mr Melvin Dick, the senior Andersen audit partner for WorldCom, testified that neither he nor any member of the Andersen team "had any inkling" of the improper accounting.
Wall Street analyst Mr Jack Grubman, who had promoted WorldCom stock, said in testimony prepared for the hearing, "I regret that I was wrong in rating WorldCom highly for too long" but insisted he was unaware of the company's true financial condition.