In the paper, the biological activity of beta-glucanes present in higher fungi, especially in Pleurotus ostreatus, was characterized. Beta-glucanes are natural polysaccharides, which stimulate the immune system mainly by activating macrophages. Moreover, they selectively reduce the level of LDL cholesterol in blood. Pleuran is contained in Pleurotus ostreatus; it is characterized by a high biological activity. Its biological activity is impacted by other compounds contained in Pleurotus ostreatus, among other things, by chitin and chitosan.