Summary. Phomopsin, tbe mycotoxin produced by Phoniopsis teptosiroiniformis, was found to bave a very bigh toxicity for sbeep. When administered as a single, subcutaneous injection over the dose range 1 25 to 98 A^g/kg body weigbt, all sheep given 37 5 «g/kg or more died. Some, tboiigh not all. died following lower doses, Ihe minimum lethal dose being 10 A'g/kg. The time course of hepatic response over 21 days after phomopsin adminisiration was followed by plasma biochemical analyses including those for some enzymes (glutamate dehydrogenase, 7-glutamyl transpeptidase. aspartale aniinoiransferase and alkaline pbosphatase). total bilirubin and the determination of bromosulphopbthalein clearance rates. Hepatobiliary impairment was apparent after all dosages of 2.5 Mg/kg and above wbile 1.25 Mg/kg approximated the 'no effecl' level.