The culture filtrate of the hyphomycete fungus Beauveria sulfurescens contains several types of toxin. They cause different symptoms when injected into the hemocoel of Galleria mellonella larvae and display different chromatographic properties. One of these toxins was partially purified by a three-step fractionation procedure. The toxic fraction contained only 0.075% of the protein of the crude extract, indicating that the toxic component was present at very low concentration in the culture filtrate of B. sulfurescens. The apparent molecular weight of the toxin was between 10 and 29 x 10(4). The toxin is a N-linked glycoprotein with oligomannosidic-type sugar chains. It displayed no proteolytic activity and appeared to be extremely potent on G. mellonella lirvae: its LD(50) was less than 10 mu g/kg of larvae. The injection of the toxin into larvae was accompanied by variable insect cuticle melanization.