Pyrenophora tritici-repentis, the causal agent of tan spot of wheat, differentially induces tan necrosis and/or chlorosis in wheat. A chlorosis-inducing, host-specific toxin, termed Ptr ToxB (formerly Ptr chlorosis toxin), was purified from the culture filtrates of a race 5 isolate of P. tritici-repentis. Partial purification was performed by 25 to 80% ammonium sulfate precipitation and passage through a carboxy-methyl-Sephadex C-25 cation exchange column. Final purification was performed by fast performance liquid chromatography, with a Mono S HR 5/5 cation exchanger, followed by size fractionation on a Superose 12 HR 10/30 column. The toxin was shown to be proteinaceous in nature, and purity was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular mass of Ptr ToxB was determined to be 6.61 kDa. The amino acid composition and partial N terminus amino acid sequence of the toxin were also obtained. Ptr ToxB was found to be heat stable, maintaining full toxic activity after 1 h at 55°C. Infiltration of toxin concentrations as low as 14 nM produced chlorosis on susceptible cultivars.