Casualties in earlier wars were due much more to diseases than to weapons. Mention has been made in history of the use of biological agents in warfare, to deny the enemy food and water and to cause disease. In the first world war chemical agents were used to cause mass casualties. Nuclear weapons were introduced in the second world war. Several countries are now involved in developing nuclear, biological and chemical weapon systems, for the mass annihilation of human beings, animals and plants, and to destroy the economy of their enemies. Recently, natural calamities and accidents in nuclear, chemical and biological laboratories and industries have caused mass instantaneous deaths in civilian population. The effects of future wars will not be restricted to uniformed persons. It is time that physicians become aware of the destructive potential of these weapons. Awareness, immediate protective measures and first aid will save a large number of persons. This series of articles will outline the medical aspects of nuclear, biological and chemical weapon systems in three parts. Part I will deal with the biological effects of a nuclear explosion. The short and long term effects due to blast, heat and associated radiation are highlighted. In Part II, the role of biological agents which cause commoner or new disease patterns is mentioned. Some of the accidents from biological warfare laboratories are a testimony to its potential deleterious effects. Part III deals with medical aspects of chemical warfare agents, which in view of their mass effects can overwhelm the existing medical resources, both civilian and military.(ABSTRACT TRUNCATED AT 250 WORDS)