A rapid blueing, indicating the presence of an extracellular oxidase, occurs when an alcoholic solution of gum guaiac is applied directly to cultures of wood-inhabiting Hymenomycetes that are associated with white rots while no color change follows its application to cultures of species that cause brown rots. Parallel tests on 133 species with the gum guaiac solution and the standard Bavendamm method, in which cultures are grown on malt agar containing gallic or tannic acid, gave identical results for nearly 90% of the species. Hence the rapid alcoholic gum guaiac method is recommended for determining extracellular oxidase production where this characteristic is used as a basis for separation in the identification of fungus cultures.