The black envelope, with its coded label 06FUM088, was handed unceremoniously to Jennifer McDuff in an examining room at the University of Maryland Medical Center. Inside was a small piece of paper with the same code and a single word:

"ACTIVE"

With that, McDuff was in -- eligible for Stage 2 in a human study conducted for one of the world's most sought-after vaccines: a defense for avian influenza. That is the virus many experts fear could mutate and cause a pandemic capable of killing millions of people. Within the hour, McDuff, a 24-year-old graduate student, was pushing up her sleeve for her next shot.

"I feel strongly about helping society," she explained matter-of-factly.

Since late March, McDuff and 149 other volunteers have been coming to the university's Center for Vaccine Development as part of the high-stakes clinical trial. Three sites nationwide were selected to test the body's response to a scientifically manipulated version of the H5N1 virus, which since 2003 has killed at least 69 men, women and children across Asia. The genesis of the test strain, in fact, was taken from a Vietnamese girl who was one of those 69.

The experimental vaccine has been deemed safe and potentially protective enough that federal officials hope to stockpile nearly 8 million doses by early next year. But one of the many continuing unknowns is the level at which it would be effective. If a lower-than-expected dosage would trigger adequate antibody production to safeguard a person and thwart transmission, the stockpile might be stretched twice as far, if not further.

"This is very important work," said James Campbell, an assistant professor of pediatrics and principal investigator for the study here. The center has tested high-profile vaccines before -- against anthrax, botulism, the plague and cholera, to name a few -- but seldom with so much public interest. People from across the country sent e-mails asking to be involved, he said. A man in England offered to fly to Maryland and stay for months if necessary.

"A lot of people," said Campbell, "are interested in moving this vaccine forward."

Among them are the center's 150 recruits, who include medical and law students, faculty and residents from the community. They were randomly and blindly assigned to one of two categories, which determined whether they would receive two doses of a saline placebo or the active vaccine. Within the active group, there was further division, with some getting as little as 7.5 micrograms of killed virus per shot and others inoculated with more than 10 times that much. (A regular flu shot, by comparison, delivers 45 micrograms blended from three viral strains.)

For all its import, their role in medical science was decidedly unassuming. The volunteers kept a one-week diary of any discomfort, side effects or complications; took their temperature daily during that same period; came back to have blood drawn; and fielded a couple phone calls. For that, they received several hundred dollars in compensation.

One of the study's subjects, Dan Monakil, is a bank trust administrator. Clinical trials are something the doctor's son inherently supports, and because his office isn't far away, he figured he could easily spare the time required. Friends and co-workers mostly applauded. "Most think it's pretty great that somebody's putting themselves out there," he said during a return visit Monday, his sweater shed for the blood draw that would mark the end of the study's Stage 1.

Stage 2 depended completely on that black-enveloped "invitation." If it revealed that Monakil had earlier been given real vaccine, he, like McDuff, could opt to continue for one more booster shot and another six months' monitoring. He opened it and got his answer: "ACTIVE." He immediately signed back on.

Medical student Nick Morin came to the center intending to do the same thing. "You have officially completed this protocol," nurse Lisa Chrisley told him as she handed him his envelope.

Morin pulled out its contents. "PLACEBO," read his little piece of paper. "Uh-oh," he said.

06FUM083 was officially done.

Nick Morin, a medical student, learns that he has been receiving a placebo during a bird flu vaccine trial. Volunteers who received the actual vaccine could opt to continue the trial.