ISOLATES of Aspergillus amstelodami from country cured hams were tested for toxicity to chick embryos and to mice. Seven of eight culture filtrates from isolates grown 30 days in yeast extract (2%)-sucrose (20%) media were lethal to chicken embryos. Six of eight culture filtrates were lethal when injected intraperitoneally into mice. Eight of eleven strains grown on sterile corn were toxic when fed to mice. Deaths occurred after only 3 days. A single isolate, toxic to both mice and chick embryos was selected for further study. Culture filtrates of this strain were lethal to male, white, weanling rats. Symptoms and gross lesions were indicative of an effect on the circulatory system. Incorporation of the mycelial mat into the diet of rats resulted in increased liver and kidney weights although deaths were not observed within 30 days. Rats fed corn inoculated with this mold had significantly lower average body weight gains than controls. Because of limited solubility of the toxic substance in most organic solvents, gel permeation chromatography was used to isolate a single toxic fraction. Purification of the compound was aticomplished by ion exchange and paper chromatography. The toxic material (Rf= 0.94) fluoresces blue when viewed under short wave UV light. The melting point of the material was 159–162°C. The UV absorption maximum of the material in 95% ethanol was 211 nm.