The government is adding an experimental anthrax treatment to the nation's bioterrorism drug stockpile, a protective protein culled from the blood of soldiers who received the anthrax vaccine.

The Food and Drug Administration would have to consent before doctors could try the immune globulin on any patients. But using this kind of immune system cell as therapy has worked for other diseases, noted an anthrax expert with the federal Centers for Disease Control and Prevention.

If someone else comes down with anthrax and needs it, discussions with FDA are far enough along that "we could rush this through," predicted the CDC's Dr. Bradley Perkins, who discussed immune globulin at a scientific meeting Monday.

Immune globulin wouldn't replace antibiotic treatment. But antibiotics kill only anthrax bacteria, not the cell-killing toxin those bacteria churn out in people's blood. The hope is that immune globulin would neutralize the toxin, Perkins said.

New anthrax treatments took on urgency with this fall's attacks-by-mail, which caused 18 confirmed cases of infection. Five people have died, and the culprit has not been found.

Thousands exposed during the attacks have taken two months of antibiotics to prevent infection, and now are being offered either anthrax vaccine or 40 extra days of drugs in case any anthrax still lurks in their lungs. Some 1,168 people have opted for extra drugs and 130 for the vaccine, CDC officials said Monday.

On a different anthrax vaccine issue, the CDC is poised to begin a new study on pre-exposure inoculations, to see if fewer doses given in a new way would be as effective and less uncomfortable than today's method.

The Pentagon hopes to inoculate millions of troops against an anthrax attack and already has vaccinated 500,000. Soldiers are supposed to get six shots over 18 months, plus an annual booster dose.

The vaccine has been used for decades and most experts consider it very safe. But it has caused controversy as some soldiers complained the shots could be linked to such long-term disorders as chronic fatigue.

This month, the CDC hopes to begin recruiting civilians to test whether as few as three shots and a booster every three years would work. Some people will get varying doses of vaccine, while others will get dummy shots.

"Probably a three-dose regimen is going to be effective," Perkins said.

The study also will test if the vaccine causes fewer injection-site side effects, such as swelling and skin nodules, if given in muscle rather than under the skin.

The study will be run at five medical centers: Emory University; University of Alabama, Birmingham; Baylor College of Medicine; the Mayo Clinic; and Walter Reed Army Medical Center.

It's unethical to expose people to anthrax to see if a vaccine really protects, so researchers first will measure levels of various immune markers, including antibodies, in the recipients' blood, and then compare them to levels found when people get the FDA-approved six-shot vaccine regimen.

Then they will compare the results to a monkey study. Monkeys given various vaccine doses will be exposed to potentially lethal anthrax doses. The amount of antibodies and other immunity markers the monkey survivors bear in their blood should provide a good clue to how well-protected people with similar blood levels would be if they encountered anthrax, CDC researcher Jairam Lingappa told a meeting at the National Academy of Sciences.

Congress ordered the academy's Institute of Medicine to oversee the CDC's study and ensure it is as scientifically rigorous as possible.