In 1943 I predicted1that the task of delivering a successful chemical attack against the American people was so great that our enemies would not consider it worth trying. With the possible exception of the nerve gases, it seems equally unlikely today that chemical agents offer our potential enemies effective weapons for long range attack. The problems of civil defense against chemical attack, therefore, can be reduced to consideration of a single group of chemical agents, the nerve gases, until such time as an enemy may be able to establish a base at or within our borders. The nerve gases were first developed by the Germans2but are now well known to both our allies and our potential enemies. They are a family of chemicals having the common property of irreversibly inhibiting the enzyme cholinesterase. They are nearly colorless, essentially odorless liquids, which yield toxic vapors on evaporation.