The lethality of Poa huecu, a plant toxic to cattle and sheep, was followed by injection of chromatographic fractions in mice. The lethal aqueous extract was administered i.p. to Rockland mice of either sex and produced motor incoordination, transient ataxia, rough hair coat, tremors and muscle contractions and, occasionally, blindness. Doses greater than 1.5 g/kg mouse were always lethal. Fractionation of this lethal extract included dialysis, column chromatography on Sephadex G-25 and fractional precipitation with ethanol. Precipitates obtained with 70% and 85% ethanol were further purified on a DEAE-cellulose column. Eight fractions were obtained, each was injected into mice. Only fractions 3-6 were toxic. Fraction 3 produced slight hepatosis and hyperemia in the liver and gliosis in the brain. None of the other tissues exhibited histological lesions. Fractions 4 and 5 caused death of all animals within 30 min to 4 hr after injection. Polyacrylamide gel electrophoresis and acid hydrolysis showed that fractions 4 and 5 contained a glycoprotein of nearly the same mol. wt (67,000-94,000). Microscopic pathology in the mice treated with the lethal glycoprotein of fraction 4 included hyperemia in the kidneys, megakaryocytes in the spleen, slight hepatosis and focal coagulative necrosis with nuclear pyknosis and karyonexis in the liver, gliosis, intracellular brain edema with axon degeneration and swollen astrocytes in the brain. These brain injuries may relate to the motor incoordination of cattle that causes a delayed righting reflex. The major monosaccharides of the lethal glycoprotein are glucose and mannose, while rhamnose, arabinose, xylose and galactose are present in low percentages. Proline and the acidic amino acids (glutamic and aspartic acids) are the most abundant in the peptidic residue.