BALTIMORE, JAN. 12 -- U.S. District Judge Norman P. Ramsey dismissed a jury today that had been freshly picked for the obstruction-of-justice trial of former Maryland senator Clarence M. Mitchell III, angrily declaring the jury could be "tainted" by sealed information in the case that Mitchell gave to reporters.

Chiding Mitchell for giving reporters information that his own lawyers had asked to be sealed involving allegations of prosecutorial misconduct and a secret government-recorded telephone conversation between Mitchell and the judge's wife, Ramsey ordered a new panel of prospective jurors to be assembled Thursday in the ill-starred case.

Because of last-minute legal maneuvering by Mitchell's attorneys, forcing Ramsey to put off starting the trial for another one or two days, the judge said it would be "intolerable . . . to keep the {current} jury any longer."

The jurors, he said, "are going to be wondering what in the world is going on here . . . . What secrets are being kept from us?" Their curiosity could force some to read news accounts about the sealed material, "and I'm not going to risk a tainted jury."

Mitchell, 48, is charged with trying to conceal from a grand jury his business ties with convicted Baltimore drug dealer Melvin (Little Melvin) Williams. He was convicted in November along with his brother, state Sen. Michael B. Mitchell, of accepting $50,000 to block a congressional probe of the scandal-scarred Wedtech Corp. in New York.

An initial mistrial was declared in the Wedtech case after a prospective juror was reported to have made racially tinged remarks to others in the jury assembly room indicating he hoped the Mitchells, who are black, would be convicted.

In the current case, Clarence Mitchell renewed allegations that the Reagan administration is trying to discredit black political leaders. Monday, he filed an affidavit by an ex-FBI informant in Atlanta, Hirsch Friedman, saying there is an "unofficial policy" by the FBI to investigate black elected officials when there is no "probable cause" to do so.

Mitchell also said he learned from prosecutors on the eve of his trial that law enforcement agents had secretly tape-recorded an April 1985 telephone conversation between him and Ramsey's wife, public relations businesswoman Tuckey Ramsey.

Armed with a court order, agents were monitoring calls at the business office of a suspected Baltimore drug dealer, according to prosecutors, and Mitchell happened to be at the office when he spoke to Tuckey Ramsey about helping him arrange a fund-raising dinner in his unsuccessful bid for Congress. Tuckey Ramsey, citing federal rules against political activity, declined to help.

Mitchell's attorney on Monday challenged Judge Ramsey's ability to sit fairly on the Mitchell trial, saying the defense might use the tape recording of his wife to help show Mitchell's innocent presence at the suspected drug dealer's office.

Ramsey turned down requests for evidentiary hearings into that issue and the alleged FBI policy. He said the phone conversation would not affect his ability to be fair, and the Atlanta affidavit lacked any connection to the Mitchells in Baltimore. He said if Mitchell gathered more credible evidence on the alleged FBI policy, he would hold a hearing later.

Mitchell's attorney went to the 4th U.S. Circuit Court of Appeals where Judge Francis D. Murnaghan Jr. upheld Ramsey's rulings.