When marine chemist John Hayes pioneered a way to scrutinize the carbon atoms in seafloor mud 15 years ago, he was trying to unravel mysteries about how dead microbes once lived. Never did he guess that sporting officials would one day use his invention to catch drug cheats.

By modifying Hayes's method to examine the carbon atoms in athletes' urine, medical researchers at the U.C.L.A. Olympic Laboratory have developed the first definitive screen for synthetic testosterone, a popular anabolic steroid banned by most sports organizations since the 1970s. The new test--known as the carbon isotope ratio (CIR) test--figures prominently in several recent, high-profile doping cases, including the disqualification of sprinter Justin Gatlin's world record in the 100 meters. It may also strip cyclist Floyd Landis of his 2006 Tour de France title.