Atropine was a chemical agent that could be used to prevent the effects of nerve gas.
Day 5 Edit
During Day 5, after the attack on the Sunrise Hills Shopping Mall, Jack Bauer asked the CTU agents for a shot of atropine for "a child under twelve" so that he could revive a young girl after her exposure to Sentox nerve gas. After Jack gave her the shot, her pulse stabilized and Jack left her in the care of the CTU medics.
Later that day, after CTU had come under attack, Jack had responded to the remaining staff that the atropine was in the armory, which was in a contaminated zone and thus of no value to the staff.