They are training to contain an invisible enemy that can kill on contact - a chemical weapon.
While the terrorist attack is simulated, the response by the FBI, Los Angeles police, fire department and the military is real.
During the three-day exercise, called Westwind '99, two nerve gas explosions "contaminate" 200 people at the local airport.
The need for this kind of preparedness is not theoretical.
In the last three months there have been half a dozen threats of exposure to deadly anthrax bacteria in the Los Angeles area alone. All were hoaxes.
"Los Angeles is a global metropolis with a porous border, close proximity to the Pacific Rim. It truly is a potential target. I think we have to be prepared," said Commander David Kalish of the Los Angeles Police Department.
So does President Clinton, who says the threat of biological and chemical weapons attacks keeps him awake at night.
The Department of Defense is holding these preparedness exercises all over the country. But so far, this is the biggest.
Observers for other cities watched and learned, and will take home lessons on how to prepare for what officials used to believe could never happen here.