Enzymatic (the action of lysozyme) and chemical (hydrogen peroxide) hydrolysis of chitosans with various degree ofacetylation (DA)--25, 17, and 1.5%--was performed. Purification and fractioning of the hydrolysis products were performed using dialysis, ultrafiltration, and gel-penetrating chromatography Low-molecular (LM) derivatives of the polysaccharide with molecular masses from 17 to 2 kDa were obtained. The study of their antiviral activity against the tobacco mosaic virus (TMV) showed that these samples inhibited the formation of local necroses induced by the virus for 50-90%. The antiviral activity of the LM chitosans significantly increased with the lowering of their polymerization degree. Furthermore, the products of the enzymatic hydrolysis possessed higher activity than the chitosan samples obtained as a result of chemical hydrolysis. It was revealed that the exhibition of the antiviral activity weakly depended on the degree of acetylation of the samples.