SummaryRecent work on the origin of soil polysaccharide and its biological stability in soil is reviewed. It is concluded that much of the constituent hexose and deoxyhexose sugars are of microbial origin, whereas the pentose sugars are derived from plant residues. The stability of soil polysaccharide in its native state is not related to its chemical composition but to its unavailability. This is caused by inaccessibility within undecomposed biological residues and to insolubility resulting from adsorption on clay, the formation of metal complexes, or tanning by soil humic substances. Complexing by metals and tanning may also inhibit enzymic hydrolysis.