Peggy Johnston did not hesitate when the ninth-grade guidance counselor asked her what career she wanted to pursue as an adult.

"I want to be a gym teacher," she replied confidently.

True to that early ambition, Johnston, 48, wears a lanyard to work every day. It's a fancy one, though, bearing the stylized image of an antibody, the symbol of the National Institute of Allergy and Infectious Diseases. In place of a whistle, a laminated photo ID card identifies Johnston as the associate director for vaccine and prevention research in the division of AIDS and the assistant director for HIV vaccines at NIAID.

Johnston oversees the U.S. effort to develop a vaccine against AIDS, the No. 1 infectious killer in the world today, and to find new medicines that might prevent the epidemic's spread. The AIDS prevention effort that she heads is the biggest in the world. She coaches a globally dispersed team of thousands of scientists, doctors and volunteers in a life-and-death scrimmage against the world's most pernicious virus.

The development of an AIDS vaccine, and the discovery of drugs to block transmission from mother to child, have proven to be extremely difficult. Cultural barriers, and distrust between AIDS scientists and people in the developing countries most affected by the epidemic, have added to the difficulties.

Yet friends and colleagues say that if anyone can chart a course through those obstacles, Johnston can. Part scientist and part diplomat, she knows how to get complicated, international research projects off the ground.

She is personally reserved--a legacy, some say, of being one of the higher-ranking gay women in the federal work force--but can also be disarmingly openhearted and candid. In countless countries around the world, Johnston has charmed, cajoled and otherwise cultivated the trust of Third World leaders to keep the global AIDS prevention effort apace.

"It's making all the pieces fit together," she says. "That's where I get my jollies."

Johnston collected some of those jollies outside the government. In 1996, after 10 years at NIAID, she left to help found the International AIDS Vaccine Initiative (IAVI), a nonprofit organization that funnels money to the AIDS vaccine effort.

At IAVI Johnston worked to create a global vaccine purchase fund that ensures a profit for any company that comes up with a useful AIDS vaccine. And she helped design a novel international research compact that has made developing countries more open to foreign vaccine researchers by promising those countries affordable access to any vaccine that emerges.

Johnston's decision to return to NIAID a year ago was the "hardest I ever made," she says. But with the federal AIDS vaccine program expanding enormously, and the unmatched concentration of expertise at the National Institutes of Health, she could not resist.

"This is where the action is now, and where it's going to be," she says.

And, she confesses, "I missed being around scientists."

That isn't to say Johnston's a laboratory nerd. Colleagues say she knows how to enjoy good Scotch with friends after scientific meetings (though dawn will inevitably find her sweating on the hotel treadmill).

Johnston also is an avid outdoorswoman--in part, she says, because being in nature reminds her of the cyclicity of all things, a perspective that helps get her through professional and personal hard times. A year and a half ago, Leslie, her partner of 13 years, died of leukemia. Johnston's father died after spending all 44 years of his adult life working in the steel mills outside Pittsburgh, where the family grew up. Her mother, a homemaker, has survived two bouts with cancer.

AIDS vaccine work follows cycles too, Johnston says, with periodic victories and lots of setbacks. Still, she has never regretted taking her guidance counselor's advice to take chemistry instead of another gym class.

After all, she concedes in mock chagrin: What a cliche she'd have been as a lesbian gym teacher.

Players

Margaret "Peggy" Johnston

Titles: Associate director for vaccine and prevention research, division of AIDS; assistant director for HIV vaccines, National Institute of Allergy and Infectious Diseases.

Age: 48.

Education: BS in chemistry, Carnegie-Mellon University; PhD in biochemistry, Tufts University; postdoctoral associate, Rega Institute for Medical Research, Leuven, Belgium.

Family: Lives with partner in Glenmont area of Silver Spring.

Career highlights: Senior staff fellow, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health; adjunct associate professor, Department of Biochemistry, Uniformed Services University of the Health Sciences, Bethesda; deputy director, division of AIDS, NIAID, NIH; scientific director and vice president for scientific affairs, International AIDS Vaccine Initiative.