The toxin from Ustilago maydis virus P4 was purified to homogeneity and characterized. The native molecular mass, using size-exclusion HPLC was estimated to be 7.2 kDa. The purified toxin was composed of a single subunit. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis under reduced and nonreduced conditions resulted in estimated molecular masses of 8.4 and 7.4 kDa, respectively. The purified toxin was found to be glycosylated when tested for carbohydrates using the phenol-sulfuric acid method, Schiff's base reagent, and a Glycan detection kit and when probed against different biotinylated lectins. Partial amino acid sequence analysis of the purified toxin indicated a free N-terminus, 16% glycine, and 23% basic amino acid residues. No homology was found to either the alpha or the beta subunit of the toxin encoded by U. maydis infected with the P6 virus.