President Carter's choice to head the Justice Department's antitrust division, Sanford Litvack, told a Senate subcommittee yesterday that he would not take part in that agency's sensitive ongoing investigation of international oil companies.

Livack removed himself from a key decision-making role in the case because he said he had represented Mobil Oil Corp. as a private attorney.

Litvack, who had been with the New York firm of Donovan, Leisure, Newton & Irvine, won speedy approval from the full Judiciary Committee yesterday, just a few hours after a confirmation hearing before the Senate antitrust subcommittee.

By a unanimous voice vote, the Judiciary Committee sent the nomination to the Senate floor for final action. There did not appear to be any opposition to Litvack's appointment to the post, which is held by John Shenefield. Shenefield has been nominated to be associate attorney general, the Justice Department's No. 3 job.

Litvack, 43, began his legal career at the antitrust division, but has been in private practice with the New York firm for most of his career.

Sen. Howard Metzenbaum (D-Ohio) chairman of the antitrust subcommittee and a frequent critic of the antitrust division under Shenefield, told Litvack that "things are not as well as they might be" in the division, citing reports of declining productivity and morale there.

Litvack said that as far as he knew morale at the division was good. "If it is good, I like it to be better," he said.

Litvack stressed his experience as an antitrust attorney and said he hoped to improve the division's efficiency.

Further, he told the Senate panel that he "would like to bring" a shared monopoly case, assuming that such a case is to be made. For some years, critics of the antitrust division have been calling for such a case and, in fact, Shenefield said he hoped to bring one during his tenure at the division. f

Litvack made no such commitment, but said he would provide the Metzenbaum subcommittee with a status report on shared monopoly investigations as promptly as possible.

Although Litvack said he had represented many corporate clients during his years of private practice, Metzenbaum specifically asked him about his work for Mobil and the possibility that he could be asked to approve legal action involving the giant oil company when the international oil price fixing probe comes to a close.

The oil probe, which Shenefield has called one of the federal government's most important cases, is an effort to look into petroleum pricing practices by international oil companies operating in the Persian Gulf.

Litvack said it would be "inappropriate ethically" for him to participate in decisions involving the oil probe and said that any matters which may have come before him, would be handled by his three deputies. The investigation is under the aegis of Donald Flexner, a deputy assistant attorney general, Litvack said.