The mental and social health consequences of exposure to biological or chemical weapons require a public health strategy. Attacks with chemical and biological weapons are likely to be accompanied by acute fear, organic mental problems, psychological responses to somatic illnesses and injuries, and long-term development of medically unexplained symptoms. The World Health Organization has developed eight principles for public mental health activities in emergencies. These principles are also valid for situations that involve biological or chemical weapons attacks. These principles pertain to preparation before the emergency, assessment, collaboration and coordination, integration into primary health care, access to services for all, training and supervision, long-term perspectives, and monitoring indicators. Following these statements of principles, the paper considers aspects of the acute emergency phase of a biological or chemical attack. It advises that general public health interventions should be complemented with a range of social and mental health interventions. In coordinating these interventions, mental health professionals should advocate and work in partnership with colleagues from other disciplines, including communication, education, community development, and disaster coordination, so as to ensure that relevant social interventions are fully implemented. The paper outlines some early social interventions, followed by descriptions of early mental health interventions. The section of the paper on the acute emergency phase is followed by a section on the post-emergency phase. Attention is given to general social interventions and general mental health interventions in this phase. 21 references and supplementary information on the experience of fear in biological and chemical warfare and the management of medically unexplained epidemic illness