Dr. Donal M. Billig, a heart surgeon at Bethesda Naval Hospital who was named chief of a surgery unit and then lost his right to practice when questions were raised about his competency, had not performed heart surgery for six years when he was hired by the Navy, according to a private consultant engaged this year to study Billig's credentials.

Billig was hired in January 1983 with the understanding that he would undergo retraining as a cardiac surgeon, said Dr. Benjamin Aaron, who testified during recent hearings that caused Billig to lose his position, and another doctor who operated extensively with Billig during a six-month retraining period at Bethesda.

The second doctor, Dr. Reginald Peniston, said he found Billig "very good" in many respects. But, he said, he recommended near the end of the training that Billig not yet be allowed to perform cardiac surgery -- operations such as coronary bypass and heart valve replacements -- without supervision. Peniston, who has since retired from Bethesda, said yesterday that his recommendation was not followed.

Within weeks after Peniston gave his views in May 1983, Billig was named a full-fledged staff member and allowed to perform operations independently, said Peniston, now head of cardio-thoracic surgery at Howard University Hospital. In June, according to naval records and the report filed by the private consultant, Billig was made head of the cardio-thoracic unit, a unit that performs both heart and chest operations, and was given approval to perform operations of both kinds.

Navy personnel would not say yesterday why Billig, now 54, was recruited as a heart surgeon. Billig, a cum laude graduate of the University of Louisville School of Medicine, had trained under renowned heart surgeon Dr. Michael DeBakey in the 1960s, taught surgery at several university medical schools and most recently had practiced at five hospitals in Pittsburgh before he was hired by the Navy.

"These types of questions fall into the category of information that is being investigated by the formal board and any comment is inappropriate at this time," said Lt. Alan Goldstein, public information officer.

A board commissioned by the Naval Medical Command is meeting this week to investigate how Billig was recruited and approved for surgery by the Navy hospital. That board's inquiry follows a decision two weeks ago by Bethesda to revoke Billig's credentials there and terminate his position as head of the surgical unit.

Billig's attorney, Lt. Neal Worden, would not comment yesterday on any questions about Billig. Billig could not be reached.

The naval hospital told Billig that, based on a review of his records and questions concerning his competence, he could no longer operate. Billig, who has been suspended from the hospital since November, has appealed that decision.

The Naval Medical Command's investigation is closed to the public. But Aaron this week discussed what he had told Bethesda officials during their investigation in March.

Aaron, who made his report for the Navy, said he pointed out that Billig had not practiced heart surgery for at least six years. The Navy made "a good review of Billig's credentials ," Aaron said. "But they didn't ask the right questions."

"He had not done cardiac surgery for six years before he went into the Navy," said Aaron, a professor and chief of cardio-thoracic surgery at George Washington University Hospital. "And nobody asked him why. Now, that's bothersome . . . . The credentialing process didn't even look into that."

Aaron, a Navy doctor for 22 years, two of which he spent at Bethesda, said his report was based on a review of Billig's resume, his cases and an interview with the doctor.

Aaron's review of Billig came about five years after a committee of doctors at a private hospital in Long Branch, N.J., had said the doctor lacked good surgical judgment, proper motivation, competence and honesty. The panel, according to records on file in a New Jersey court, ruled that Billig could not operate unless he was supervised and asked him to leave.

Monmouth Medical Center's evaluation of Billig is contained in a 1980 court case brought by Billig against the hospital in an unsuccessful attempt to remain at the hospital. Several other doctors in the court case reviewed Billig's performance and found it proper. Billig surrendered his license in New Jersey in June 1981.

Hospital representatives said they forwarded a "full and complete disclosure" on Billig to Bethesda in March 1983, when the naval hospital requested information about the doctor. Hospital representatives said they made the same disclosure to state officials in New Jersey.

Aaron said Bethesda Naval personnel reviewed the record about the incident at Monnmouth "quite thoroughly, but Dr. Billig put together quite a rebuttal that portrayed it as a personality clash." That, Aaron said, satisfied Navy personnel reviewing Billig's record.

"They didn't view that decredentialing process at Monmouth as a significant professional act," said Aaron. "But a decredentialing procedure is an extremely unusual event in a good surgeon's career. To just write it off as a group of doctors being jealous doesn't seem quite right."

Peniston, who said he supervised much of Billig's work during the retraining session at Bethesda, said yesterday he was asked to submit a military fitness report, a routine written report, about Billig's performance near the end of May 1983. Peniston described the report yesterday as a "glowing report with qualifications -- the qualifications being what his capabilities were in cardiac surgery."

"My feeling then was that he was still in the process of getting trained and he should be supervised in terms of his operating skills," said Peniston, who was a cardio-thoracic surgeon for 11 years with the Navy and retired in June 1983. " . . . I found out that at the beginning of June, he had been given privileges to operate independently."

That month, Billig was also made chief of the cardio-thoracic unit at Bethesda.

Peniston said yesterday that he had given what he considered a "fair report" to Bethesda officials.

"My biggest criticism was his surgical techniques were not adequate to operate alone," said Peniston, emphasizing his review was completed in May 1983 before officials cited Billig for problems. Billig was first suspended from the Bethesda hospital for a month in October 1983 for problems related to his work and then again in November 1984. The last suspension led to his dismissal hearing.

Asked about his concerns over Billig's surgery, Peniston said, "He had not learned cardiac surgery at a time when many refinements had come through. Cardiac surgery is hard to practice if you're trying to practice like you did 10 years ago."