Millionaire developer Dominic F. Antonelli Jr. and D.C. mayoral aide Joseph P. Yeldell, who were convicted last month of bribery and conspiracy, were granted a new trial yesterday by a federal judge because of a juror's failure to divulge that once she had a checking account at a bank founded by Antonelli.

Central to yesterday's ruling by Judge Gerhard A. Gesell was a surprise disclosure at a mid-afternoon hearing that the juror, Diane P. Jones, 27, had maintained a small personal checking account during 1975 and 1976 at Madison National Bank, of which Antonelli was a founder, major stockholder and director.

At the start of the trial early last month, all prospective jurors were asked last month, all prospective jurors were asked as a group in court whether they had ever had any dealings with Madison. Although several others responded that they had accounts at Madison. Jones, an Amtrak accounting clerk, remained silent.

Judge Gesell ruled that Jones, by failing to reply to this and one other question during the jury selection procedures, had "made intentional nondisclosures" and had, "in effect, misled counsel" for Antonelli and Yeldell. Gesell described the juror's failure to respond to the questions as amounting to "reckless" conduct.

"The Madison bank is at the center of the case," Gesell said as he announced his order for a new trial in U.S. District Court. "I think the failure [by the juror] to disclose . . . is highly material." Madison bank officials, as Gesell noted, testified during the trial, and Madison bank loans to Yeldell were cited in criminal charges against Yeldell and Antonelli. Jones, moreover, had maintained her Madison account during years when key events in the trial occured.

Antonelli, 56, a real estate developer and parking company owner, and Yeldell, 46, who is on unpaid leave as a top aide to Mayor Walter E. Washington, had been scheduled to be sentenced Thursday on the bribery and conspiracy charges.

The frequently outspoken, powerful city official and the influential; publicity-shy millionaire had been convicted on Oct. 24 of conspiring corruptly to arrange an allegedly lucrative D.C. government lease of a drab, two-story building at 60 Florida Ave. NE from a partnership controlled by Antonelli.

In exchange for Yeldell's help in securing the $5.6 million, 20-year lease, Antonelli was found by the jury to have secretly given Yeldell a $33,000 personal loan after helping Yeldell obtain a series of short-term loans, initially amounting to $21,500, from Madison National Bank.

The jury had deliberated only about 3 hours and 40 minutes before reaching its verdict at the end of the three-week trial. Unless further legal developments occur, a new trial appears likely to be scheduled before Judge Gesell early next year.

Neither Antonelli nor Yeldell was present in court yesterday when Gesell announced his ruling. Reached at home, Yeldell, the former Human Resources, would only say. "I don't have any comment." Yeldell's defense lawyer, John A. Shorter Jr., described himself, however, as "just completely overjoyed."

Other supporters of Yeldell expressed delight at Gesell's ruling. Sam Jordan, Mayor Washington's chief community trouble-shooter, described it as "the best news I've heard this year."

Edward Bennett Williams, Antonelli's chief defense lawyer, whose efforts to obtain a new trial led to yesterday's order, declined to comment, as was his practice throughout the trial. Asked whether Antonelli would have any comment, Williams replied, "He never says anything."

The assistant U.S. attorneys who had prosecuted Antonelli and Yeldell huddled in their office to discuss the judge's order, and declined to comment on it. They had repeatedly opposed Antonelli's and Yeldell's requests for a new trial.

The court proceedings were the outgrowth of a stormy controversy that eventually became the biggest political scandal in the District of Columbia's nearly four years of home-rule government. Allegations made against Yeldell and Antonelli have been cited by some of Mayor Washington's supporters as a key factor in the mayor's defeat in last September's Democratic primary, won by Mayor-elect Marion Barry.

The controversy, which stemmed partly from disclosures by The Washington Post nearly two years ago about Yeldell's dealings with Antonelli, led to an 18-month grand jury investigation and last month's trial. The costs of the proceedings, including sequestering the jury a local motel, were estimated yesterday to amount to tens of thousands of dollars.