No known domestic or international terrorist organizations have signaled a change in tactics from traditional forms of terrorism such as bombings and kidnappings to the use of CB weapons. Yet, the gravity of the potential use of such weapons, coupled with the capability demonstrated by perpetrators of the Tokyo, Japan, subway attack, require that law enforcement agencies be prepared for such incidents. For fanatical groups and individuals, the attraction of CB weapons lies in their relative ease and economy of production, compared to other methods of mass destruction. In addition, legitimate civil uses for CB components and related technology limit their control and make the substances relatively easy to acquire. In general, the definition of a CB incident includes any agent that might cause mass casualties by the release or use of a hazardous material. Chemical agents can produce damaging or lethal effects in people, animals, and plants, while infectious agents that cause illnesses in people are used to create biological weapons. In responding to CB incidents, law enforcement agencies should develop contingency plans that incorporate various emergency service capabilities, including evacuation, medical treatment, containment, decontamination, criminal investigation, and prosecution. Although the Federal Bureau of Investigation is the lead agency for coordinating the Federal response to CB incidents, State and local law enforcement agencies have significant roles to play in mitigating the damage caused by a CB event. The effectiveness of any contingency plan hinges on preventing potential attacks before CB agents can be disseminated. 14 endnotes and 3 photographs