Pyrenophora graminea culture filtrates contain a high mol. wt phytotoxic compound which, upon infiltration into host leaves, reproduces symptoms of barley leaf stripe. Extracts from P. graminea cell walls had similar toxic activity. The toxic compound was purified by selective precipitation in acetone, dialysis and gel-filtration on Sephacryl S-300-HR resin. The non-purified phytotoxic compound had a strong tendency to aggregate as fibrilar material or in crystal-like structures. In gel-filtration it was separated into two fractions of, respectively, 250-350 kDa and 55 kDa. The minor fraction originated from the dissociation of the major one. Both fractions contained a carbohydrate and a protein moiety. The importance of the glycosyl moiety (compound of fucose, glucuronic acid, glucose and galactose) for toxic activity was indicated by toxic compound inactivation after periodate oxidation and after β-glucuronidase digestion. Treatment of the toxic compound with different proteases did not affect toxic activity. The protein moiety was removed from the toxin by β-climination without affecting toxic activity. The toxic compound is host-specific. Antiserum raised against a purified fraction of the toxin, was able to detect the toxic compound in vivo in P. graminea -infected barley plants but not in healthy plants.