The pectic polysaccharide named potamogetonan (PN) was obtained using extraction of the leaves and stems of the common pondweed Potamogeton natans L. by an aqueous ammonium oxalate. The purified potamogetonan PN-300 was obtained using membrane ultrafiltration of PN and proved to be pectin with a molecular weight of 300 kDa. The capacity of potamogetonan PN-300 to prevent inflammation was assessed using a carrageenan paw edema test in mice. Oral administration of PN-300 24 h prior to induction of inflammation was found to reduce edema formation in a dose-related manner. The maximal effect of PN-300 was observed at 1 h after carrageenan injection (60% reduction of footpad swelling) and was comparable to that of indomethacin. The delayed edema (5 h) was less affected by pre-administration of PN-300 (33% reduction). PN-300 was found to improve the survival of mice subjected to a lethal dose of LPS. The anti-endotoxemic effect of PN-300 was shown to be mediated by decreased TNF-alpha and IL-1beta and increased IL-10 production.Thus, a pectin named potamogetonan PN-300 was isolated from P. natans and was shown to possess a preventive antiinflammatory effect following oral administration.