The nation's only vaccine approved against anthrax has never been a best-seller. The only civilians who ever needed it were ranchers, hide handlers, veterinarians and the few scientists who worked with the deadly soil bacterium.

Sept. 11 changed all that. Doctors' offices and travel clinics around the country have been swamped with requests for the vaccine amid concern that the tragedies in New York and Washington might foreshadow a bioterrorist attack. More than 1,000 people in the past two weeks have tried to get shots directly from the vaccine's maker, BioPort of Lansing, Mich. Callers there are being shunted to a recorded message that reflects what doctors everywhere are saying:

"All the stockpile that currently exists is owned by the Department of Defense. At this time there is no opportunity for any commercial sales."

That reality has infuriated some. "After all, we are all now soldiers in the war trenches of our offices, hospitals and backyards," said Robert Gray, an Atlanta area health care worker. He noted that Iraq, the Soviet Union and perhaps other nations have made weapons loaded with the deadly spores.

Yet BioPort's vaccine was until two weeks ago perhaps the most shunned and controversial shot ever produced. For years it has been at the center of a political, medical and legal struggle, with more than 400 members of the military opting to quit or be court-martialed rather than roll up their sleeves.

Indeed, the history of the vaccine suggests that the nation faces some difficult decisions as it tries to prepare for what is widely believed to be the No. 1 biowarfare threat today.

The Food and Drug Administration has repeatedly cited BioPort for violations at its manufacturing plant, including contamination problems and suspicious changes made to quality assurance records. So severe have those problems been that the agency has blocked the release of all batches for several years, contributing to a severe shortage that was causing problems for the military long before the public wanted a share of the doses.

BioPort may eventually overcome its manufacturing problems -- the company says it will apply for FDA licensure of an upgraded plant in mid-October. But questions remain as to whether the FDA itself is in violation of the law by allowing the military to use the vaccine against inhaled anthrax when the shot's usefulness has been proven only for a different version of the disease.

In May, two Air Force officers filed a federal class action suit, contending that the vaccine should be classified as "investigational" for inhaled anthrax and should be offered only to volunteers who sign informed-consent documents clearly stating they are subjects in an experiment.

Pentagon leaders also are apparently engaged in a heated battle among themselves over the vaccine. Undersecretaries of Defense E.C. "Pete" Aldridge Jr. and David S.C. Chu, who were reviewing the anthrax vaccine situation for Defense Secretary Donald H. Rumsfeld, sent a memo to Rumsfeld on Aug. 10 saying they would make their final recommendations about BioPort by Sept. 1.

Language in the memo supports long-standing rumors that they may recommend canceling the contract with BioPort and developing a new facility for the military's vaccine needs.

Their final Sept. 1 recommendation has not been made public. But on Aug. 30 Gen. Henry H. Shelton, chairman of the Joint Chiefs of Staff, sent his own preemptory memo to Rumsfeld.

That memo, which was obtained by The Washington Post, urges Rumsfeld to keep the BioPort vaccine "as the centerpiece of our defense" against anthrax. Sources inside and outside the military, speaking on the condition of anonymity, said the memo appears to be an effort to derail the undersecretaries' review. They say it reflects the joint chiefs' position that the vaccine will give them political, if not medical, cover if U.S. troops are attacked with anthrax.

Inhaled anthrax is fatal in 80 percent of unvaccinated people. Antibiotics for the disease, such as ciprofloxacin (brand name Cipro), which have been selling in record quantities in the past two weeks, are largely useless by the time symptoms arise.

The vaccine was approved in 1970, based on a study that showed it protected against "cutaneous" anthrax, a form of the disease that occurs when spores infect the skin. The vaccine was never tested against inhaled anthrax in people (animal studies had hinted, but did not prove, efficacy, and human tests cannot be conducted today for ethical reasons). Nonetheless, about half a million members of the military were ordered to take the shots to protect against inhaled anthrax during the Gulf War.

In 1996, BioPort applied to the FDA for a change in the vaccine's approval to deem it officially protective against inhaled anthrax. That application is still pending, but in 1997 the military embarked on a massive program to vaccinate all 2.4 million active and reserve military personnel.

Those plans have shrunk over the years, largely as a result of dwindling supplies caused by BioPort's ongoing failure to pass FDA requirements. About 520,000 members of the service have been inoculated to date.

Critics of the vaccine say no one should be forced to get the shot because, in their view, its safety and efficacy against inhalation anthrax are still in question. The U.S. General Accounting Office has agreed -- among other problems, it concluded, the current vaccine does not have the same chemical formulation as that approved in 1970. A scathing report by the House Government Reform Committee also agreed.

It remains to be seen whether the military will use the heightened threat of terrorism to justify a new push for mass vaccination of troops. Alternatively, soldiers could be given the choice of opting out of what they see as an unfair experiment, or the president could invoke a 1998 law that allows him to compel soldiers to submit to experimental shots.

Neither of those options is politically attractive, observers said. And given the current climate of fear, it may be that -- in a twist no one had predicted -- the real challenge will be making enough of the vaccine to satisfy public demand.