The presence of a polyphenoloxidase in wheat stem rust uredospores has been demonstrated. The enzyme was released by the germinating spores together with phenolic substrates, resulting in the formation of phenol oxidation products in the surrounding medium. The substrate specificity of the enzyme was investigated, and gallic acid was found to give an unusually high oxidation rate. Pyrogallic acid and catechol were also rapidly oxidized. Toxicity tests have been carried out with phenol oxidation products on germinating spores and it has been shown that toxicity depends on the stage of oxidation. Short-term oxidations led to the formation of highly toxic compounds; longer periods of oxidation converted these into harmless products, whereas very long periods of oxidation resulted again in the production of toxic substances. The spores also contained a quinone reductase which apparently holds the phenolics in a reduced state until they are released during germination or during the infection process. The possible role of the phenol – phenoloxidase – quinone reductase system in the host–parasite relations of the wheat – stem rust complex is discussed.