During the past two decades, research on animal, plant, and microbial toxins has expanded rapidly, and new and exciting information has appeared to clarify both the clinical and therapeutic aspects of intoxication and, even more impor- tant, to help us understand more exactly the structure and the mode of action of toxins on a molecular basis. Because of the interdisciplinary nature of toxin research, it is of vital impor- tance that workers specializing in its different aspects should make a particular effort to exchange and keep up with new developments in this rapidly-expanding field. This has been one of the main purposes of the series of international meetings that have been held over the past ten years. The increasing interest in the results of toxin research can be explained partly by the growing general preoccupation with pollution and its toxic effects, which means that more anq more specialists in other fields are finding it necessary to keep in touch with current research into naturally occurring toxins. The papers in these volumes were presented at the Fourth International Symposium on Animal, Plant and Microbial Toxins, organized by the Inter- national Society on Toxicology, held in Tokyo in September, 1974. The editors have decided to include both original reports and review articles, arranged according to disciplines. Editing has been kept to the minimum necessary for adequate comprehension of the materials.