Cultivation of two commercial Pleurotus ostreatus (oyster mushroom) strains was performed in plastic bags. Tree leaves appeared to be an excellent growth substrate for the conversion into fruiting bodies with biological efficiency of 108-118%. The level of enzyme activity was strongly regulated during the life cycle of mushrooms. However, despite the quantitative variations, each strain had a similar pattern of enzyme accumulation in fermentation of both substrates. Laccase and MnP activities were high during substrate colonization and declined rapidly during fruiting body development. On the contrary, in substrate colonization P. ostreatus expressed comparatively low activity of hydrolases. When primordia appeared, the activity of these enzymes sharply increased. Both cellulase and xylanase activity peaked at the mature fruiting body stage. When mushrooms shifted to the vegetative growth, the activity of ligninolytic enzymes again gradually increased, whereas the activity of hydrolases decreased.