Billy Carter testified for two hours and 15 minutes before a federal grand jury today on alleged attempts by the Libyan government to influence the Carter administration improperly.

The president's brother was accompanied only by his lawyer, Henry Ruth.

Carter testified before a 23-member grand jury probing charges that improper influence was used on Carter administration officials in an attempt to obtain delivery of eight C130 military transport planes to the Libyan government.

A U.S. Senate subcommittee has already heard testimony on Billy Carter's ties with Libya from Carter and Henry R. (Randy) Coleman, a friend.

Carter has received $220,000 from the Libyan government. He has described the payments as loans and has denied he exerted influence on the White House.

Asked if he had any comment after leaving the grand jury room, Billy Carter said, "I answered every question truthfully."

In October 1978, Coleman was briefed on the Libya plane sale by Morris Draper, a deputy assistant secretary of state. The briefing came shortly after Coleman and the president's brother returned from a trip to Libya.

In other news relating to the Billy Carter investigation today, a senator said in Washington he believes President Carter allowed his brother to set up a White House meeting with a Libyan official to help Billy's Libyan ties.

Sen. Richard Lugar (R-Ind.) said he believes the president considered the meeting harmless and wanted, out of compassion, to help Billy cement some sort of trade deal with Libya.

Billy Carter denied at a Senate hearing last week that Libyans ever asked him to try to influence U.S. policy for them or that he ever did.

Lugar, a member of the special Senate Judiciary subcommitte investigating Billy Carter, suggested his theory at the hearing last week and expanded on it at a news conference today.

"Although I feel this was a misuse of the foreign policy apparatus by the president, there's a mixed motivation involved here," Lugar said. "Billy Carter testified that his affairs at the warehousfe and life generally were tragic after the election of the president."

Lugar said he believes that is why the president had Billy Carter set up a meeting last November between the president's national security adviser, Zbigniew Brzezinski, and Libya's U.S. emissary, Ali Houderi.

The purpose of the meeting was to ask Libyan aid in efforts to free the U.S. hostages in Iran, but Lugar sasid the president surely knew that Libya could not help free the hostages.

"I believe the president is a compassionate person who saw that his brother was in very grave trouble [financially]," the senator said.

"It seems to me that the president felt that by incorporating Billy Carter in this very small vestige, as he saw it, of foreign policy that this was a fairly harmless activity," he said.

Meanwhile, in New York, the Libyan mission to the United Nations confirmed that its chief delegate, Mansur Kikhia, is available to testify in the Billy Carter affair before any appropriate American body.

In a statement, the mission spoke of "false allegations, unfounded accusations and rumors against Libyan officials."

Kikhia was ready to give evidence in public before any appropriate congressional committee or subcommittee dealing with the matter "to clarify the facts."

The mission said it had contacted congressional authorities to inform them of Kikhia's availability.