Chitin and chitosan are nitrogenous polysaccharides. The structure of the chitin molecule is similar to that of cellulose but it is composed of the units of 2-acetylamino-2-deoxy-D-glucopyranose bound (1 --> 4) by a glycosidic bond. In contrast to chitin, chitosan amino groups are not mostly acetylated. These polysaccharides occur particularly in the fungal kingdom, in invertebrate animals, in some brown algae, and very negligibly in higher plants. Great attention has been paid to chitin and chitosan in recent years. Of great interest are their antihypercholesterolemic and antihyperlipidemic activities which have been experimentally demonstrated. In practice there already exist some medicinal preparations and food products with medicinal and preventive properties against some risk factors of atherosclerosis. Also the anticoagulative and anti-aggregative effects of sulfated derivatives of chitosan are not negligible. The research in this field is, however, at the initial stage. Both polysaccharides have been introduced into pharmaceutical technology where they can be employed as very good drug vehicles. Promising results have been achieved in experiments with antineoplastics (5-fluorouracil and methotrexate), with nonsteroidal antiphlogistics (ibuprofen) and others. It is considered very advantageous that chitin and chitosan possess low toxicity, allergize only a little, and exert moderate immunostimulating effects. Both polysaccharides are metabolized by lysosome. Chitin is obtained from the cuticles of sea animals which are waste products from food industry. The technologies of obtaining chitin from lower fungi (e.g. the genus Aspergillus) have been worked out. Chitosan is obtained by deacetylation of chitin. Chitin and chitosan are polysaccharides with ever-extending use. Increasing attention is paid to them also due to the fact that they are contained in a large extent in food fungi, e.g. Pleurotus ostreatus.