The pharmacological properties of plant polysaccharides are reviewed and original experimental data on the properties of water-soluble polysaccharides isolated from Acorus calamus L. are presented. The possibility of using plant (in particular, Acorus calamus) polysaccharides to increase the effectiveness of anticancer treatment of transferred tumors and to reduce the toxic effects of cytostatic treatment on the normal cells of blood, liver, and epithelium of thin intestine in experimental animals has been demonstrated.