WASHINGTON--The judge in the Iran-Contra case ruled today that Oliver L. North, John M. Poindexter and two other defendants each be tried separately, so they can defend themselves using each other's congressional testimony given under a grant of limited immunity.

U.S. District Judge Gerhard A. Gesell asked Independent Counsel Lawrence E. Walsh to decide whether he wants to try North or Poindexter in the first of four trials.

The other two defendants, Richard V. Secord and Albert Hakim, would be tried later.

Gesell also said the trials should be held in Washington, rejecting a defense argument that the pretrial publicity has damaged the defendants' right to have a fair trial.

The defendants had urged a separate trial on grounds that the immunized testimony included information that would prove their innocence. For instance, Secord has said that testimony by North would show that money from U.S. arms sales to Iran belonged to Secord's business enterprise rather than the U.S. government as the indictment charged.

Gesell said that "any defendant should have considerable leeway to use indirectly...and possibly directly immunized testimony while defending the charges against him."

He said the right to a fair trial "cannot be preserved if any two defendants are tried together."

Walsh is barred from using the defendants' immunized congressional testimony as evidence against them even though the defendants can use it themselves.

Outside the courthouse, Walsh declined to comment on the judge's decision except to say that "everyone realizes that four trials are more burdensome than one but the judge feels that that's a necessary requirement to protect the constitutional rights of all four defendants."

Walsh declined to say whether North of Poindexter would be tried first. Gesell gave the prosecutor until Friday to make his decision.

Gesell said defendants at criminal trials are "afforded the powerful weapon of cross-examination, an essential part of our adversarial system."

"Cross-examination is an effective means of ensuring the truth will prevail before a jury. It's a precious constitutional right," the judge said.

"Each of the three defendants who gave immunized testimony revealed information which one or more of the defendants considers helpful to his own defense if not exonerating some aspect" of the case, Gesell said.

"Each defendant has contemplated using immunized testimony of other witnesses," Gesell said.

The judge said Congress had a legitimate interest in conducting the hearings but he noted that the questioning of North, Poindexter and Hakim was "in a highly accusatory context spiked with accusation of criminal impropriety."