Pleurotus ostreatus mycelium produced extracellular cellulases when grown on ground wheat straw. No cellulase activity was detectable when glucose or aqueous extracts of vegetable material were used as substitutes for straw in culture media. The specific activity of excreted cellulases did not vary significantly by increasing the straw concentration from 1 to 6%. Chromatographic fractionation of extracellular proteins gave rise to five fractions with cellulolytic activity. The hydrolytic properties of these partially purified fractions were analysed by using several substrates (carboxymethylcellulose, cellobiose, p-nitrophenyl-beta-D-cellobioside, p-nitrophenyl-beta-D-lactoside). The results indicate that the cellulase system of P. ostreatus includes at least a beta-glucosidase, two endocellulases, an exoglucohydrolase and an exocellobiohydrolase.