It is 10 a.m. and Dr. Anthony S. Fauci, nattily dressed in a dark blue pin-striped suit, rushes into an immunology lab on the 11th floor of the National Institutes of Health hospital and dons a white coat.

The transition is quick and complete -- the busy administrator of a research institute with a $ 500 million-plus budget is again an attending physician on Ward 11 East and West, worried about his patients. Grand rounds are about to begin.

For the next few hours, Fauci will see an array of patients: some with rare immunological diseases such as Wegener's granulomatosis, and AIDS patients with a skin cancer called Kaposi's sarcoma who are participating in a new government study of the drug AZT.

When the clock strikes 12, Fauci reverts to his pin stripes and returns to his job as director of the National Institute of Allergy and Infectious Diseases (NIAID).

At 45, he is the youngest of the 12 institute directors at the National Institutes of Health, headquarters of the nation's federal biomedical research effort. Although infectious diseases have long taken a back seat to chronic ailments such as heart disease and cancer, the AIDS epidemic has propelled Fauci's institute into an increasingly prominent role in studying one of the most deadly diseases of all time.

Fauci has become a frequent public spokesman on acquired immune deficiency syndrome and also coordinates AIDS research for NIH.

When Fauci became institute director on Nov. 2, 1984, he also kept his duties as director of the Laboratory of Immunoregulation and physician of record for many research patients. He continues to publish scientific papers; his 78-page curriculum vitae lists 555 papers to his credit.

Fauci has managed all these jobs remarkably well. Colleagues who admire his unusual success as a physician-scientist-administrator give him reviews worthy of television's Dr. Kildare:

"Tony has one major vice: he works too hard. He is such a capable individual it's hard not to ask him to do as much as people ask him to do. He does everything amazingly well," said Dr. Walter Dowdle, head of the Centers for Disease Control's Center for Infectious Diseases and former AIDS coordinator for the Public Health Service.

"He's an extraordinary person. He's a first-rate scientist, an excellent physician, a very well-organized administrator and a genuinely decent human being," said Dr. Sheldon M. Wolff, chief of medicine at Tufts University and a 17-year NIH veteran who was Fauci's mentor when Fauci first came on board.

"He's done a great job. He's given the institute new purpose," said Dr. Vincent T. DeVita Jr., National Cancer Institute chief, who urged Fauci to take the directorship.

Fauci is a transplanted New Yorker who was born in Brooklyn on Christmas Eve 1940, graduated from College of the Holy Cross in Worcester, Mass., in 1962, and received his medical degree in 1966 from Cornell University Medical College in New York City.

After an internship and residency there, he came to NIH in 1968 as an NIAID fellow and returned to New York for a one-year stint as chief resident. Fauci came back to Bethesda in 1972 to work at NIAID and has been there since.

While many scientists choose a career either at the "bench," working in a laboratory on basic research, or in clinical work, developing experimental therapies, Fauci has done both. He has divided his time between basic research on how the immune system works and treatment of diseases that may involve the immune system, the body's defense mechanism.

He made a name for himself developing a cure for several fatal diseases few people have heard of, including Wegener's granulomatosis. It is an auto-immune disease in which patients develop a hypersensitivity that inflames their blood vessels, lungs and airways. A drug regimen developed by Fauci and others made it "imminently curable," he says.

Fauci's many years "dissecting the immune system" put him in the right place when a mysterious new fatal disease was first recognized in mid-1981. "Everything mushroomed," Fauci recalls, when he gradually shifted the focus of his laboratory toward the frightening illness that became known as AIDS.

There was "an extra fascination that we must be dealing with something that is brand new. How often in medicine are physicians and scientists faced with something totally new? I had a feeling of awe and some smoldering anxiety that we had a larger problem ahead of us," he says.

Fauci began to treat AIDS patients with some highly experimental approaches, including a dramatic procedure involving bone-marrow transplants from a healthy identical twin to a twin brother with AIDS.

His laboratory also identified the nature of the defect in the immune cells, known as T4 helper cells, that helped explain what makes AIDS so deadly. "Our contribution was saying this was the one set of cells that is responsible for orchestrating the immune system," says Fauci.

The lion's share of NIH research money and the national limelight was initially on the National Cancer Institute and the major discovery in the laboratory of Dr. Robert Gallo that a specific virus carried in the blood and bodily fluids was the cause of AIDS. NCI researchers such as Dr. Samuel Broder also took a lead in early studies of potential anti-AIDS drugs such as AZT.

Under Fauci's leadership, however, the role of the rejuvenated infectious diseases institute has grown, particularly in large-scale testing of new AIDS drugs.

The NIAID budget for AIDS jumped from $ 297,000 in fiscal 1982 to $ 63 million in 1986, and $ 146 million is estimated for 1987, accounting for more than one-quarter of the institute's projected $ 545 million budget and nearly 60 percent of NIH's funding for AIDS. The NIAID will spend more than twice as much this year on AIDS work as the NCI.

Within NIH, the budget shifts and Fauci's increasingly high public profile have inevitably begun to draw a few negative reviews from scientists who respect him but feel he is taking too much personal credit.

His insistence on keeping his laboratory is also a potential danger, said one scientist, because "it brings all sorts of suspicion," however unfounded, that his loyalties to his lab might interfere with his duties as an impartial institute director.

Some of his closest associates also worry that despite Fauci's seemingly boundless energy, he may burn out if he keeps up his relentless schedule indefinitely. His work day -- in by 6:45 a.m. and often at the office until well after 9 p.m. -- is long even by Washington workaholic standards.

Besides his professional duties, he has a new responsibility as the father of 2-month-old Jennifer. (He met Christine Grady, who became his wife, on the job: she is a clinical nurse specialist at the institute.)

But Fauci manages to run several miles a day on his lunch hour.

"There is something in him that has to show people that he can do it all," said one colleague. "Is he Superman? About as close as you could find . . . . He is doing it all at what may be a tremendous personal cost. He really overworks himself."

"My main job is director of the institute, there's no question about that," says Fauci, adding with a laugh that if something had to go, "I'd give up sleep."

He said he is able to do so much because of the "star" team that helps manage his various areas of responsibility -- people such as his assistant, Dr. James C. Hill, and his top laboratory investigators, Dr. H. Clifford Lane and Dr. Randi Leavitt.

Fauci said he is a better administrator because he stays close to the lab and clinical work at the core of the institute. Although he is a man always in a hurry, he has a special patience for patients, stopping on rounds to have a word with each of the day's unusual "cases."

He looks with pleasure on a 24-year-old man who is in for a checkup, cured several years earlier from a rare vasculitis, or blood vessel disease, as a result of Fauci's research.

"It's terrific. Keep it up," says the doctor to his blushing patient. Outside, Fauci adds, "That made my day . . . . Of all of the things I've done, this guy makes it worth it."