The studies reported in this chapter were carried out by the author in the Russian Confederate Republic of U.S.S.R., in the Orenburg district in order to elucidate the factors conducive to the occurrence and development of the very severe, so-called Alimentary Toxic Aleukia (ATA) disease. These findings have been supplemented with recent work in Israel in the Laboratory of Mycology and Mycotoxicology of the Hebrew University of Jerusalem by studying the toxicity of authentic ATA-producing Fusarium species to animals and plants and also the chemical nature of their toxic compounds. The author hopes that this published material will be of great interest and use to biologists, chemists, toxicologists, physicians, veterinarians, and pharmacologists.