Athletes aren't the only ones racing against the clock this week in Beijing. A team of skilled scientists is working 24 hours a day at a drug-testing lab in Beijing's Olympic Sports Center, analyzing approximately 4,500 blood and urine samples for banned substances. Their work is part of an ever-evolving arms race between scientists and sports cheats who try to stay one step ahead of the latest detection methods.

On Monday, the International Olympic Committee (IOC) announced the first athlete to fail a drug test in Beijing: Spanish cyclist Maria Isabel Moreno, who tested positive for the red-blood-cell-boosting hormone erythropoietin (EPO). But IOC president Jacques Rogge has predicted that 30 to 40 athletes will test positive during the games.

EPO is used therapeutically to treat anemia, but it also boosts blood oxygenation in healthy people, and it has proved troublesome for scientists to detect. For one thing, traces of the drug are quickly eliminated from the body. "When the drug is gone, the urine test becomes negative, but the effect of the drug lasts longer and the athlete is still enhanced," says Don Catlin, founder of Anti-Doping Research, a nonprofit research institute based in Los Angeles that is helping oversee drug testing in Beijing during the games. "Therefore, athletes game the test, trying to figure out the dosing regimens that will keep them beneath the radar."

In an attempt to catch those athletes out, the Olympic antidoping lab has dramatically stepped up testing compared with previous games, conducting 1,000 more tests than in Athens in 2004 and double the number at the Sydney games in 2000. That increase comes largely from greater numbers of tests per sample, rather than from an increase in the number of samples collected.

The IOC and the World Anti-Doping Agency (WADA) are also developing new testing techniques, although they won't give details about any new tests that they plan to run at this year's Olympics. "We need the elements of secrecy to try to be ahead of the game," saysCatlin.

This secrecy won WADA a dramatic victory at the Tour de France last month. Its drug-testing lab caught several cyclists using a longer-lasting form of EPO called CERA. Soon after the athletes were caught, it was revealed that the agency had been working with Swiss drugmaker Roche to develop a test to detect CERA while the drug was still being tested by the U.S. pharmaceutical company Amgen.

Unfortunately, generic versions of the drug are popping up all over the world. And because each has a chemical composition slightly different from the original version, scientists must design a new test for each variety. (Scientists detect the different forms of EPO using a standard laboratory technique called electrophoresis, which separates molecules based on their charge. The man-made versions have a different charge than the peptide made naturally in the body.)

Catlin says that many more versions of EPO are likely to emerge, as well as a related class of drugs called next-generation erythropoiesis-stimulating agents, or ESAs. One of those, Hematide, is already in late-stage clinical trials. "As soon as that's out, it will find its way into the hands of sportsmen and -women," Catlin says.