Republicans on the Senate Judiciary Committee agreed yesterday to a restructuring of the panel to abolish the antitrust subcommittee and revive a semblance of the old internal security subcommittee.

Sen. Strom Thurmond (R-S.C.), who will become committee chairman, announced creation of nine subcommittees. That compares to the current seven, and several will have different functions. Thurmond said he will claim no subcommittee chair, so all nine other committee Republicans, including four freshman, can head one each.

One of the freshman, Jeremiah Denton of Alabama, a retired admiral elected with the support of the fundamentalist Moral Majority, will head a new subcommittee on security and terrorism, Thurmond said in a statement.

A Thurmond aide said the new subcommittee would keep watch over communist activities within the United States and would have added jurisdiction over terrorism. In addition, according to a committee source, it will have jurisdiction over mutiny, espionage, counterfeiting, the FBI and the Drug Enforcement Administration.

For years the committee had a subcommittee on internal security headed by former senator James O. Eastland of Mississippi that conducted free-wheeling anticommunist investigations to the dismay of civil libertarians. It was disbanded four years ago as was a similar committee in the House.

The antitrust subcommittee, which for years provided a forum for liberals to investigate big business, will be disbanded and its jurisdiction folded into the full committee where it can be overseen by Thurmond.

The subcommittee on the Constitution, which handles civil rights legislation and constitutional amendments, will be headed by Orrin Hatch (R-Utah) who is currently leading a Senate fillibuster against a civil rights bill to end racial discrimination in housing. Paul Laxalt (R-Nev.), President-elect Ronald Reagan's man in the Senate, will be in charge of regulatory reform. Charles McC. Mathias Jr. (R-Md.) will head the subcommittee on criminal law.

Thurmond said the Republican members agreed to reduce the committee budget by 25 percent, leaving $1,250,000 less than outgoing Chairman Edward M. Kennedy (D-Mass.) had to work with this year.