Catalase strongly inhibits the toxicity of the cotton phytoalexin desoxyhemigossypol (dHG) cotton against Verticillium dahliae. Since catalase destroys hydrogen peroxide (H₂O₂) it appears H₂O₂ is an essential compound in expression of the toxic action of dHG. H₂O₂ is produced during the decomposition of dHG. The actual toxic entity may be H₂O₂ itself or the hydroxy radical (HOú) derived from it. Superoxide (O₂ú ) does not appear to be involved in the toxic activity since superoxide dismutace does not decrease toxicity. Reduced glutathione also inhibits dHG toxicity presumably by retarding the oxidation of dHG.